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Sunday, May 18, 2025

Steven Pinker

From Wikipedia, the free encyclopedia
Steven Pinker
Pinker in 2023
Born
Steven Arthur Pinker

September 18, 1954 (age 70)
Montreal, Quebec, Canada
Nationality
  • Canadian
  • American
Education
Notable work
Spouses

(m. 1980; div. 1992)

Ilavenil Subbiah
(m. 1995; div. 2006)

(m. 2007)
RelativesSusan Pinker (sister)

Awards
Scientific career
Fields
Institutions
ThesisThe representation and manipulation of three-dimensional space in mental images (1979)
Doctoral advisorStephen Kosslyn
Websitestevenpinker.com

Steven Arthur Pinker (born September 18, 1954) is a Canadian-American cognitive psychologist, psycholinguist, popular science author, and public intellectual. He is an advocate of evolutionary psychology and the computational theory of mind.

Pinker is the Johnstone Family Professor of Psychology at Harvard University. He specializes in visual cognition and developmental linguistics, and his experimental topics include mental imagery, shape recognition, visual attention, regularity and irregularity in language, the neural basis of words and grammar, and childhood language development. Other experimental topics he works on are the psychology of cooperation and of communication, including emotional expression, euphemism, innuendo, and how people use "common knowledge", a term of art meaning the shared understanding in which two or more people know something, know that the other one knows, know the other one knows that they know, and so on.

Pinker has written two technical books that proposed a general theory of language acquisition and applied it to children's learning of verbs. In particular, his work with Alan Prince published in 1989 critiqued the connectionist model of how children acquire the past tense of English verbs, positing that children use default rules, such as adding -ed to make regular forms, sometimes in error, but are obliged to learn irregular forms one by one.

Pinker is the author of nine books for general audiences. The Language Instinct (1994), How the Mind Works (1997), Words and Rules (2000), The Blank Slate (2002), and The Stuff of Thought (2007) describe aspects of psycholinguistics and cognitive science, and include accounts of his own research, positing that language is an innate behavior shaped by natural selection and adapted to our communication needs. Pinker's The Sense of Style (2014) is a general language-oriented style guide. Pinker's book The Better Angels of Our Nature (2010) posits that violence in human societies has generally declined over time, and identifies six major trends and five historical forces of this decline, the most important being the humanitarian revolution brought by the Enlightenment and its associated cultivation of reason. Enlightenment Now (2018) further argues that the human condition has generally improved over recent history because of reason, science, and humanism. The nature and importance of reason is also discussed in his next book Rationality: What It Is, Why It Seems Scarce, Why It Matters (2021).

In 2004, Pinker was named in Time's "The 100 Most Influential People in the World Today", and in the years 2005, 2008, 2010, and 2011 in Foreign Policy's list of "Top 100 Global Thinkers". Pinker was also included in Prospect Magazine's top 10 "World Thinkers" in 2013. He has won awards from the American Psychological Association, the National Academy of Sciences, the Royal Institution, the Cognitive Neuroscience Society, and the American Humanist Association. He delivered the Gifford Lectures at the University of Edinburgh in 2013. He has served on the editorial boards of a variety of journals, and on the advisory boards of several institutions. Pinker was the chair of the Usage Panel of the American Heritage Dictionary from 2008 to 2018.

Biography

Pinker was born in Montreal, Quebec, in 1954, to a middle-class secular Jewish family in an English-speaking community. His grandparents immigrated to Canada from Poland and Romania in 1926, and owned a small necktie factory in Montreal. His father, Harry, worked in real estate and was a lawyer. His mother, Roslyn, was originally a homemaker, but later became a guidance counsellor and a high-school vice-principal. In an interview, Pinker described his mother as "very intellectual" and "an intense reader [who] knows everything". His brother, Robert, worked for the Canadian government for several decades as an administrator and a policy analyst, while his sister, Susan Pinker, is a psychologist and writer who authored The Sexual Paradox and The Village Effect. Susan is also a columnist for The Wall Street Journal.

Pinker graduated from Dawson College in 1973. He graduated from McGill University in 1976 with a Bachelor of Arts in psychology, then did doctoral studies in experimental psychology at Harvard University under Stephen Kosslyn, receiving a PhD in 1979. He did research at the Massachusetts Institute of Technology (MIT) for a year, then became a professor at Harvard and later, Stanford University.

From 1982 until 2003 Pinker taught at the Department of Brain and Cognitive Sciences at MIT, was the co-director of the Center for Cognitive Science (1985–1994), and eventually became the director of the Center for Cognitive Neuroscience (1994–1999), taking a one-year sabbatical at the University of California, Santa Barbara, in 1995–96. Since 2003 he has served as the Johnstone Family Professor of Psychology at Harvard, and between 2008 and 2013 he also held the title of Harvard College Professor in recognition of his dedication to teaching. In the early 2010s, he gave lectures as a visiting professor at the New College of the Humanities, a private college in London.

Pinker married Nancy Etcoff in 1980 and they divorced in 1992; he married again in 1995 and again divorced. His third wife, whom he married in 2007, is the novelist and philosopher Rebecca Goldstein. He has two stepdaughters, the novelist Yael Goldstein Love and the poet Danielle Blau. Pinker adopted atheism at 13, but at various times was a "cultural Jew". Pinker is an avid cyclist.

Linguistic career

Pinker in 2011

Pinker's research on visual cognition, begun in collaboration with his thesis adviser, Stephen Kosslyn, showed that mental images represent scenes and objects as they appear from a specific vantage point (rather than capturing their intrinsic three-dimensional structure), and thus correspond to the neuroscientist David Marr's theory of a "two-and-a-half-dimensional sketch." He also showed that this level of representation is used in visual attention, and in object recognition (at least for asymmetrical shapes), contrary to Marr's theory that recognition uses viewpoint-independent representations.

In psycholinguistics, Pinker became known early in his career for promoting computational learning theory as a way to understand language acquisition in children. He wrote a tutorial review of the field followed by two books that advanced his own theory of language acquisition, and a series of experiments on how children acquire the passive, dative, and locative constructions. These books were Language Learnability and Language Development (1984), in Pinker's words "outlin[ing] a theory of how children acquire the words and grammatical structures of their mother tongue", and Learnability and Cognition: The Acquisition of Argument Structure (1989), in Pinker's words "focus[ing] on one aspect of this process, the ability to use different kinds of verbs in appropriate sentences, such as intransitive verbs, transitive verbs, and verbs taking different combinations of complements and indirect objects". He then focused on verbs of two kinds that illustrate what he considers to be the processes required for human language: retrieving whole words from memory, like the past form of the irregular verb "bring", namely "brought"; and using rules to combine (parts of) words, like the past form of the regular verb "walk", namely "walked".

In 1988 Pinker and Alan Prince published a critique of a connectionist model of the acquisition of the past tense (a textbook problem in language acquisition), followed by a series of studies of how people use and acquire the past tense. This included a monograph on children's regularization of irregular forms and his popular 1999 book, Words and Rules: The Ingredients of Language. Pinker argued that language depends on two things: the associative remembering of sounds and their meanings in words, and the use of rules to manipulate symbols for grammar. He presented evidence against connectionism, where a child would have to learn all forms of all words and would simply retrieve each needed form from memory, in favour of the older alternative theory, the use of words and rules combined by generative phonology. He showed that mistakes made by children indicate the use of default rules to add suffixes such as "-ed": for instance 'breaked' and 'comed' for 'broke' and 'came'. He argued that this shows that irregular verb-forms in English have to be learnt and retrieved from memory individually, and that the children making these errors were predicting the regular "-ed" ending in an open-ended way by applying a mental rule. This rule for combining verb stems and the usual suffix can be expressed as Vpast → Vstem + d, where V is a verb and d is the regular ending. Pinker further argued that since the ten most frequently occurring English verbs (be, have, do, say, make ... ) are all irregular, while 98.2% of the thousand least common verbs are regular, there is a "massive correlation" of frequency and irregularity. He explains this by arguing that every irregular form, such as 'took', 'came' and 'got', has to be committed to memory by the children in each generation, or else lost, and that the common forms are the most easily memorized. Any irregular verb that falls in popularity past a certain point is lost, and all future generations will treat it as a regular verb instead.

In 1990 Pinker, with Paul Bloom, published a paper arguing that the human language faculty must have evolved through natural selection. The article provided arguments for a continuity-based view of language evolution, contrary to then-current discontinuity-based theories that see language as suddenly appearing with the advent of Homo sapiens as a kind of evolutionary accident. This discontinuity-based view was prominently argued by two main authorities, linguist Noam Chomsky and Stephen Jay Gould. The paper became widely cited and created renewed interest in the evolutionary prehistory of language, and has been credited with shifting the central question of the debate from "did language evolve?" to "how did language evolve?" The article also presaged Pinker's argument in The Language Instinct.

In 2006 Pinker provided to Alan Dershowitz, a personal friend of Pinker's who was Jeffrey Epstein's defense attorney, Pinker's own interpretation of the wording of a federal law pertaining to the enticement of minors into illegal sex acts via the internet. Dershowitz included Pinker's opinion in a letter to the court during proceedings that resulted in a plea deal in which all federal sex trafficking charges against Epstein were dropped. In 2019, Pinker stated that he was unaware of the nature of the charges against Epstein, and that he engaged in an unpaid favor for his Harvard colleague Dershowitz, as he had regularly done. He stated in an interview with BuzzFeed News that he regrets writing the letter. Pinker says he never received money from Epstein and met with him three times over more than a dozen years, and said he could never stand Epstein and tried to keep his distance.

Popularization of science

Human cognition and natural language

Pinker at CSICon in 2018, hosted by the Committee for Skeptical Inquiry

Pinker's 1994 The Language Instinct was the first of several books to combine cognitive science with behavioral genetics and evolutionary psychology. It introduces the science of language and popularizes Noam Chomsky's theory that language is an innate faculty of mind, with the controversial twist that the faculty for language evolved by natural selection as an adaptation for communication. Pinker criticizes several widely held ideas about language – that it needs to be taught, that people's grammar is poor and getting worse with new ways of speaking, the Sapir–Whorf hypothesis that language limits the kinds of thoughts a person can have, and that other great apes can learn languages. Pinker sees language as unique to humans, evolved to solve the specific problem of communication among social hunter-gatherers. He argues that it is as much an instinct as specialized adaptative behavior in other species, such as a spider's web-weaving or a beaver's dam-building.

Pinker states in his introduction that his ideas are "deeply influenced" by Chomsky; he also lists scientists whom Chomsky influenced to "open up whole new areas of language study, from child development and speech perception to neurology and genetics" – Eric Lenneberg, George Miller, Roger Brown, Morris Halle and Alvin Liberman. Brown mentored Pinker through his thesis; Pinker stated that Brown's "funny and instructive" book Words and Things (1958) was one of the inspirations for The Language Instinct.

There has been debate about the explanatory adequacy of the theory. By 2015, the linguistic nativist views of Pinker and Chomsky had a number of challenges on the grounds that they had incorrect core assumptions and were inconsistent with research evidence from psycholinguistics and child language acquisition. The reality of Pinker's proposed language instinct, and the related claim that grammar is innate and genetically based, has been contested by linguists such as Geoffrey Sampson in his 1997 book, Educating Eve: The 'Language Instinct' Debate. Sampson argues that "while it may seem attractive to argue the nature side of the 'nature versus nurture' debate, the nurture side may better support the creativity and nobility of the human mind." Sampson denies there is a language instinct, and argues that children can learn language because people can learn anything. Others have sought a middle ground between Pinker's nativism and Sampson's culturalism.

The assumptions underlying the nativist view have also been questioned in Jeffrey Elman's Rethinking Innateness: A Connectionist Perspective on Development, which defends the connectionist approach that Pinker attacked. In his 1996 book Impossible Minds, the machine intelligence researcher Igor Aleksander calls The Language Instinct excellent, and argues that Pinker presents a relatively soft claim for innatism, accompanied by a strong dislike of the 'Standard Social Sciences Model' or SSSM (Pinker's term), which supposes that development is purely dependent on culture. Further, Aleksander writes that while Pinker criticises some attempts to explain language processing with neural nets, Pinker later makes use of a neural net to create past tense verb forms correctly. Aleksander concludes that while he doesn't support the SSSM, "a cultural repository of language just seems the easy trick for an efficient evolutionary system armed with an iconic state machine to play."

Pinker lecturing to humanists in the United Kingdom (2018)

Two other books, How the Mind Works (1997) and The Blank Slate (2002), broadly surveyed the mind and defended the idea of a complex human nature with many mental faculties that are genetically adaptive (Pinker is an ally of Daniel Dennett and Richard Dawkins in many disputes surrounding adaptationism). Another major theme in Pinker's theories is that human cognition works, in part, by combinatorial symbol-manipulation, not just associations among sensory features, as in many connectionist models. On the debate around The Blank Slate, Pinker called Thomas Sowell's book A Conflict of Visions "wonderful", and explained that "The Tragic Vision" and the "Utopian Vision" are the views of human nature behind right- and left-wing ideologies.

In Words and Rules: the Ingredients of Language (1999), Pinker argues from his own research that regular and irregular phenomena are products of computation and memory lookup, respectively, and that language can be understood as an interaction between the two. "Words and Rules" is also the title of an essay by Pinker outlining many of the topics discussed in the book. Critiqueing the book from the perspective of generative linguistics Charles Yang, in the London Review of Books, writes that "this book never runs low on hubris or hyperbole". The book's topic, the English past tense, is in Yang's view unglamorous, and Pinker's attempts at compromise risk being in no man's land between rival theories. Giving the example of German, Yang argues that irregular nouns in that language at least all belong to classes, governed by rules, and that things get even worse in languages that attach prefixes and suffixes to make up long 'words': they can't be learnt individually, as there are untold numbers of combinations. "All Pinker (and the connectionists) are doing is turning over the rocks at the base of the intellectual landslide caused by the Chomskian revolution."

In The Stuff of Thought (2007), Pinker looks at a wide range of issues around the way words related to thoughts on the one hand, and to the world outside ourselves on the other. Given his evolutionary perspective, a central question is how an intelligent mind capable of abstract thought evolved: how a mind adapted to Stone Age life could work in the modern world. Many quirks of language are the result.

Pinker is critical of theories about the evolutionary origins of language that argue that linguistic cognition might have evolved from earlier musical cognition. He sees language as being tied primarily to the capacity for logical reasoning, and speculates that human proclivity for music may be a spandrel – a feature not adaptive in its own right, but that has persisted through other traits that are more broadly practical, and thus selected for. In How the Mind Works, Pinker reiterates Immanuel Kant's view that music is not in itself an important cognitive phenomenon, but that it happens to stimulate important auditory and spatio-motor cognitive functions. Pinker compares music to "auditory cheesecake", stating that "As far as biological cause and effect is concerned, music is useless". This argument has been rejected by Daniel Levitin and Joseph Carroll, experts in music cognition, who argue that music has had an important role in the evolution of human cognition. In his book This Is Your Brain On Music, Levitin argues that music could provide adaptive advantage through sexual selection, social bonding, and cognitive development; he questions the assumption that music is the antecedent to language, as opposed to its progenitor, noting that many species display music-like habits that could be seen as precursors to human music.

Pinker has also been critical of "whole language" reading instruction techniques, stating in How the Mind Works, "... the dominant technique, called 'whole language,' the insight that [spoken] language is a naturally developing human instinct has been garbled into the evolutionarily improbable claim that reading is a naturally developing human instinct." In the appendix to the 2007 reprinted edition of The Language Instinct, Pinker cited Why Our Children Can't Read by cognitive psychologist Diane McGuinness as his favorite book on the subject and noted:

One raging public debate involving language went unmentioned in The Language Instinct: the "reading wars," or dispute over whether children should be explicitly taught to read by decoding the sounds of words from their spelling (loosely known as "phonics") or whether they can develop it instinctively by being immersed in a text-rich environment (often called "whole language"). I tipped my hand in the paragraph in [the sixth chapter of the book] which said that language is an instinct but reading is not. Like most psycholinguists (but apparently unlike many school boards), I think it's essential for children to be taught to become aware of speech sounds and how they are coded in strings of letters.

The Better Angels of Our Nature

Detail from "Mars" in Das Mittelalterliche Hausbuch, c. 1475 – 1480. Pinker used the image in The Better Angels of Our Nature to illustrate violence in the Middle Ages.

In The Better Angels of Our Nature, published in 2011, Pinker argues that violence, including tribal warfare, homicide, cruel punishments, child abuse, animal cruelty, domestic violence, lynching, pogroms, and international and civil wars, has decreased over multiple scales of time and magnitude. Pinker considers it unlikely that human nature has changed. In his view, it is more likely that human nature comprises inclinations toward violence and those that counteract them, the "better angels of our nature". He outlines several "major historical declines of violence" that all have their own social/cultural/economic causes.

Response to the book was divided. Many critics found its arguments convincing and its synthesis of a large volume of historical evidence compelling. This and other aspects drew criticism, including the use of deaths per capita as a metric, Pinker's liberal humanism, the focus on Europe, the interpretation of historical data, and its image of indigenous people. Archaeologist David Wengrow summarized Pinker's approach to archaeological science as "a modern psychologist making it up as he goes along".

English writing style in the 21st century

In his seventh popular book, The Sense of Style: The Thinking Person's Guide to Writing in the 21st Century (2014), Pinker attempts to provide a writing style guide that is informed by modern science and psychology, given that William Strunk wrote The Elements of Style in 1918, nearly a full century prior to Pinker’s publication.

Views

Pinker and Nils Brose speaking at a neuroscience conference

Pinker identifies as a liberal who is critical of some aspects of the political left. He supports same-sex marriage, a universal basic income, the legalization of drugs, the taxation of carbon, and the abolition of capital punishment. Pinker is a strong supporter of the Democratic Party. However, Pinker has argued that the far-left has created an atmosphere of intellectual intolerance on college campuses and elsewhere, and helped form the Council on Academic Freedom at Harvard to combat what he described as an epidemic of censorship at universities. He was a signatory of the Letter on Justice and Open Debate which argued that discussion of political issues was being silenced by a widespread "intolerance of opposing views, a vogue for public shaming and ostracism, and a tendency to dissolve complex issues into a binding moral certainty."

Pinker has sharply criticized social conservatives, such as former chairman of the President's Council on Bioethics Leon Kass, for opposing stem cell research, arguing that their moral views were mere expressions of disgust that were obstructing treatments that could save millions of lives.

Pinker is a frequent participant in public debates surrounding the contributions of science to contemporary society. Social commentators such as Ed West, author of The Diversity Illusion, consider Pinker important and daring in his willingness to confront taboos, as in The Blank Slate. According to West, the doctrine of tabula rasa remained accepted "as fact, rather than fantasy" a decade after the book's publication. West describes Pinker as "no polemicist, and he leaves readers to draw their own conclusions".

In January 2005 Pinker defended comments by then-President of Harvard University Lawrence Summers. Summers had speculated that in addition to differing societal demands and discrimination, "different availability of aptitude at the high end" may contribute to gender gaps in mathematics and science. In a debate between Pinker and Elizabeth Spelke on gender and science, Pinker argued in favor of the proposition that the gender difference in representation in elite universities was "explainable by some combination of biological differences in average temperaments and talents interacting with socialization and bias".

In January 2009 Pinker wrote an article about the Personal Genome Project and its possible impact on the understanding of human nature in The New York Times. He discussed the new developments in epigenetics and gene-environment interactions in the afterword to the 2016 edition of his book The Blank Slate. Pinker has been criticised for using the data of scientific racists (on subjects unrelated to race), such as the blogger Steven Sailer, with journalist Angela Saini stating that "for many people, Pinker's willingness to entertain the work of individuals who are on the far right and white supremacists has gone beyond the pale". Pinker has stated that he condemns racism.

In a November 2009 article for The New York Times, Pinker wrote a mixed review of Malcolm Gladwell's essays, criticizing his analytical methods. Gladwell replied, disputing Pinker's comments about the importance of IQ on teaching performance and by analogy the effect, if any, of draft order on quarterback performance in the National Football League. Advanced NFL Stats addressed the issue statistically, siding with Pinker and showing that differences in methodology could explain the two men's differing opinions.

In an appearance for BBC World Service's Exchanges At The Frontier programme, an audience member questioned whether the virtuous developments in culture and human nature (documented in The Better Angels of Our Nature) could have expressed in our biology either through genetic or epigenetic expression. Pinker responded that it was unlikely since "some of the declines have occurred far too rapidly for them to be explicable by biological evolution, which has a speed limit measured in generations, but crime can plummet in a span of 15 years and some of these humanitarian reforms like eliminating slavery and torture occurred in, say, 50 years". Helga Vierich and Cathryn Townsend wrote a critical review of Pinker's sweeping "civilizational" explanations for patterns of human violence and warfare in response to a lecture he gave at Cambridge University in September 2015.

In his 2018 book Enlightenment Now, Pinker posited that Enlightenment rationality should be defended against attacks from both the political left and political right. In a debate with Pinker, post-colonial theorist Homi Bhabha said that Enlightenment philosophy had immoral consequences such as inequality, slavery, imperialism, world wars, and genocide, and that Pinker downplayed them. Pinker argued that Bhabha had perceived the causal relationship between Enlightenment thinking and these sources of suffering "backwards", responding in part that "The natural state of humanity, at least since the dawn of civilization, is poverty, disease, ignorance, exploitation, and violence (including slavery and imperial conquest). It is knowledge, mobilised to improve human welfare, that allows anyone to rise above this state." In a 2019 story in Current Affairs, proprietor Nathan Robinson criticised Pinker, saying that he misrepresents his critics' arguments against his work.

In 2020, an open letter to the Linguistic Society of America requesting the removal of Pinker from its list of LSA Fellows and its list of media experts was signed by hundreds of academics. The letter accused Pinker of a "pattern of drowning out the voices of people suffering from racist and sexist violence, in particular in the immediate aftermath of violent acts and/or protests against the systems that created them", citing as examples six of Pinker's tweets. Pinker said in reply that through this letter, he, and more importantly, younger academics with less protection, were being threatened by "a regime of intimidation that constricts the theatre of ideas." Several academics criticized the letter and expressed support for Pinker. The executive committee of the Linguistic Society of America declined to strike Pinker from its lists and issued a response letter stating that "It is not the mission of the Society to control the opinions of its members, nor their expression."

In December 2024, Pinker resigned from the board of honorary members of the Freedom from Religion Foundation over what he and several colleagues viewed as the Foundation's "quasi-religious" approach to defining gender. His resignation was followed by those of Jerry Coyne and Richard Dawkins.

Awards and distinctions

Pinker was named one of Time's 100 most influential people in the world in 2004 and one of Prospect and Foreign Policy's 100 top public intellectuals in both years the poll was carried out, 2005 and 2008; in 2010 and 2011 he was named by Foreign Policy to its list of top global thinkers. In 2016, he was elected to the National Academy of Sciences.

His research in cognitive psychology has won the Early Career Award (1984) and Boyd McCandless Award (1986) from the American Psychological Association, the Troland Research Award (1993) from the National Academy of Sciences, the Henry Dale Prize (2004) from the Royal Institution of Great Britain, and the George Miller Prize (2010) from the Cognitive Neuroscience Society. He has also received honorary doctorates from the universities of Newcastle, Surrey, Tel Aviv, McGill, Simon Fraser University and the University of Tromsø. He was twice a finalist for the Pulitzer Prize, in 1998 and in 2003. Pinker received the Golden Plate Award of the American Academy of Achievement in 1999. On May 13, 2006, he received the American Humanist Association's Humanist of the Year award for his contributions to public understanding of human evolution. For 2022 he was awarded the BBVA Foundation Frontiers of Knowledge Award in the category of "Humanities and Social Sciences".

From 2008 to 2018, Pinker chaired the Usage Panel of the American Heritage Dictionary. He wrote the essay on usage for the fifth edition of the Dictionary, published in 2011. In February 2001, Pinker, "whose hair has long been the object of admiration, and envy, and intense study",[122] was nominated by acclamation as the first member of the Luxuriant Flowing Hair Club for Scientists (LFHCfS) organized by the Annals of Improbable Research.

Group selection

From Wikipedia, the free encyclopedia
image of lekking blackcock, an instance of social behaviour
Early explanations of social behaviour, such as the lekking of blackcock, spoke of "the good of the species". Blackcocks at the Lek watercolour and bodycolour by Archibald Thorburn, 1901.

Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene.

Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behavior of animals could affect their survival and reproduction as groups, speaking for instance of actions for the good of the species. In the 1930s, Ronald Fisher and J. B. S. Haldane proposed the concept of kin selection, a form of biological altruism from the gene-centered view of evolution, arguing that animals should sacrifice for their relatives, and thereby implying that they should not sacrifice for non-relatives. From the mid-1960s, evolutionary biologists such as John Maynard Smith, W. D. Hamilton, George C. Williams, and Richard Dawkins argued that natural selection acted primarily at the level of the gene. They argued on the basis of mathematical models that individuals would not altruistically sacrifice fitness for the sake of a group unless it would ultimately increase the likelihood of an individual passing on their genes. A consensus emerged that group selection did not occur, including in special situations such as the haplodiploid social insects like honeybees (in the Hymenoptera), where kin selection explains the behaviour of non-reproductives equally well, since the only way for them to reproduce their genes is via kin.

In 1994 David Sloan Wilson and Elliott Sober argued for multi-level selection, including group selection, on the grounds that groups, like individuals, could compete. In 2010 three authors including E. O. Wilson, known for his work on social insects especially ants, again revisited the arguments for group selection. They argued that group selection can occur when competition between two or more groups, some containing altruistic individuals who act cooperatively together, is more important for survival than competition between individuals within each group. A large group of ethologists conceded that while inclusive fitness may be debatable, it was still a useful theory in practice. However, the vast majority of behavioural biologists have not been convinced by renewed attempts to revisit group selection as a plausible mechanism of evolution.

Early developments

Charles Darwin developed the theory of evolution in his book, Origin of Species. Darwin also made the first suggestion of group selection in The Descent of Man that the evolution of groups could affect the survival of individuals. He wrote, "If one man in a tribe... invented a new snare or weapon, the tribe would increase in number, spread, and supplant other tribes. In a tribe thus rendered more numerous there would always be a rather better chance of the birth of other superior and inventive members."

Once Darwinism had been accepted in the modern synthesis of the mid-twentieth century, animal behavior was glibly explained with unsubstantiated hypotheses about survival value, which was largely taken for granted. The naturalist Konrad Lorenz had argued loosely in books like On Aggression (1966) that animal behavior patterns were "for the good of the species", without actually studying survival value in the field. Richard Dawkins noted that Lorenz was a "'good of the species' man" so accustomed to group selection thinking that he did not realize his views "contravened orthodox Darwinian theory". The ethologist Niko Tinbergen praised Lorenz for his interest in the survival value of behavior, and naturalists enjoyed Lorenz's writings for the same reason. In 1962, group selection was used as a popular explanation for adaptation by the zoologist V. C. Wynne-Edwards. In 1976, Richard Dawkins wrote a well-known book on the importance of evolution at the level of the gene or the individual, The Selfish Gene.

Honeybee social behaviour can be explained by their inheritance system
Social behavior in honeybees is explained by kin selection: their haplodiploid inheritance system makes workers very closely related to their queen (centre).

From the mid-1960s, evolutionary biologists argued that natural selection acted primarily at the level of the individual. In 1964, John Maynard Smith, C. M. Perrins (1964), and George C. Williams in his 1966 book Adaptation and Natural Selection cast serious doubt on group selection as a major mechanism of evolution; Williams's 1971 book Group Selection assembled writings from many authors on the same theme.

It was at that time generally agreed that this was the case even for eusocial insects such as honeybees, which encourages kin selection, since workers are closely related.

Kin selection and inclusive fitness theory

Experiments from the late 1970s suggested that selection involving groups was possible. Early group selection models assumed that genes acted independently, for example a gene that coded for cooperation or altruism. Genetically based reproduction of individuals implies that, in group formation, the altruistic genes would need a way to act for the benefit of members in the group to enhance the fitness of many individuals with the same gene. But it is expected from this model that individuals of the same species would compete against each other for the same resources. This would put cooperating individuals at a disadvantage, making genes for cooperation likely to be eliminated. Group selection on the level of the species is flawed because it is difficult to see how selective pressures would be applied to competing/non-cooperating individuals.

Kin selection between related individuals is accepted as an explanation of altruistic behavior. R.A. Fisher in 1930 and J.B.S. Haldane in 1932 set out the mathematics of kin selection, with Haldane famously joking that he would willingly die for two brothers or eight cousins. In this model, genetically related individuals cooperate because survival advantages to one individual also benefit kin who share some fraction of the same genes, giving a mechanism for favoring genetic selection.

Inclusive fitness theory, first proposed by W. D. Hamilton in the early 1960s, gives a selection criterion for evolution of social traits when social behavior is costly to an individual organism's survival and reproduction. The criterion is that the reproductive benefit to relatives who carry the social trait, multiplied by their relatedness (the probability that they share the altruistic trait) exceeds the cost to the individual. Inclusive fitness theory is a general treatment of the statistical probabilities of social traits accruing to any other organisms likely to propagate a copy of the same social trait. Kin selection theory treats the narrower but simpler case of the benefits to close genetic relatives (or what biologists call 'kin') who may also carry and propagate the trait. A significant group of biologists support inclusive fitness as the explanation for social behavior in a wide range of species, as supported by experimental data. An article was published in Nature with over a hundred coauthors.

One of the questions about kin selection is the requirement that individuals must know if other individuals are related to them, or kin recognition. Any altruistic act has to preserve similar genes. One argument given by Hamilton is that many individuals operate in "viscous" conditions, so that they live in physical proximity to relatives. Under these conditions, they can act altruistically to any other individual, and it is likely that the other individual will be related. This population structure builds a continuum between individual selection, kin selection, kin group selection and group selection without a clear boundary for each level. However, early theoretical models by D.S. Wilson et al. and Taylor showed that pure population viscosity cannot lead to cooperation and altruism. This is because any benefit generated by kin cooperation is exactly cancelled out by kin competition; additional offspring from cooperation are eliminated by local competition. Mitteldorf and D. S. Wilson later showed that if the population is allowed to fluctuate, then local populations can temporarily store the benefit of local cooperation and promote the evolution of cooperation and altruism. By assuming individual differences in adaptations, Yang further showed that the benefit of local altruism can be stored in the form of offspring quality and thus promote the evolution of altruism even if the population does not fluctuate. This is because local competition among more individuals resulting from local altruism increases the average local fitness of the individuals that survive.

Another explanation for the recognition of genes for altruism is that a single trait, group reciprocal kindness, is capable of explaining the vast majority of altruism that is generally accepted as "good" by modern societies. The phenotype of altruism relies on recognition of the altruistic behavior by itself. The trait of kindness will be recognized by sufficiently intelligent and undeceived organisms in other individuals with the same trait. Moreover, the existence of such a trait predicts a tendency for kindness to unrelated organisms that are apparently kind, even if the organisms are of another species. The gene need not be exactly the same, so long as the effect or phenotype is similar. Multiple versions of the gene—or even meme—would have virtually the same effect. This explanation was given by Richard Dawkins as an analogy of a man with a green beard. Green-bearded men are imagined as tending to cooperate with each other simply by seeing a green beard, where the green beard trait is incidentally linked to the reciprocal kindness trait.

Multilevel selection theory

Kin selection or inclusive fitness is accepted as an explanation for cooperative behavior in many species, but the scientist David Sloan Wilson argues that human behavior is difficult to explain with only this approach. In particular, he claims it does not seem to explain the rapid rise of human civilization. Wilson has argued that other factors must also be considered in evolution. Wilson and others have continued to develop group selection models.

Early group selection models were flawed because they assumed that genes acted independently; but genetically based interactions among individuals are ubiquitous in group formation because genes must cooperate for the benefit of association in groups to enhance the fitness of group members. Additionally, group selection on the level of the species is flawed because it is difficult to see how selective pressures would be applied; selection in social species of groups against other groups, rather than the species entire, seems to be the level at which selective pressures are plausible. On the other hand, kin selection is accepted as an explanation of altruistic behavior. Some biologists argue that kin selection and multilevel selection are both needed to "obtain a complete understanding of the evolution of a social behavior system".

In 1994, David Sloan Wilson and Elliott Sober argued that the case against group selection had been overstated. They considered whether groups can have functional organization in the same way as individuals, and consequently whether groups can be "vehicles" for selection. They do not posit evolution on the level of the species, but selective pressures that winnow out small groups within a species, e.g. groups of social insects or primates. Groups that cooperate better might survive and reproduce more than those that did not. Resurrected in this way, Wilson & Sober's new group selection is called multilevel selection theory.

In 2010, Martin Nowak, C. E. Tarnita and E. O. Wilson argued for multi-level selection, including group selection, to correct what they saw as deficits in the explanatory power of inclusive fitness. A response from 137 other evolutionary biologists argued "that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature".

David Sloan Wilson compared multilevel selection to a nested set of Russian dolls
David Sloan Wilson and Elliott Sober's 1994 Multilevel Selection Model, illustrated by a nested set of Russian matryoshka dolls. Wilson himself compared his model to such a set.

Wilson compared the layers of competition and evolution to nested sets of Russian matryoshka dolls. The lowest level is the genes, next come the cells, then the organism level and finally the groups. The different levels function cohesively to maximize fitness, or reproductive success. The theory asserts that selection for the group level, involving competition between groups, must outweigh the individual level, involving individuals competing within a group, for a group-benefiting trait to spread.

Multilevel selection theory focuses on the phenotype because it looks at the levels that selection directly acts upon. For humans, social norms can be argued to reduce individual level variation and competition, thus shifting selection to the group level. The assumption is that variation between different groups is larger than variation within groups. Competition and selection can operate at all levels regardless of scale. Wilson wrote, "At all scales, there must be mechanisms that coordinate the right kinds of action and prevent disruptive forms of self-serving behavior at lower levels of social organization." E. O. Wilson summarized, "In a group, selfish individuals beat altruistic individuals. But, groups of altruistic individuals beat groups of selfish individuals."

Wilson ties the multilevel selection theory regarding humans to another theory, gene–culture coevolution, by acknowledging that culture seems to characterize a group-level mechanism for human groups to adapt to environmental changes.

MLS theory can be used to evaluate the balance between group selection and individual selection in specific cases. An experiment by William Muir compared egg productivity in hens, showing that a hyper-aggressive strain had been produced through individual selection, leading to many fatal attacks after only six generations; by implication, it could be argued that group selection must have been acting to prevent this in real life. Group selection has most often been postulated in humans and, notably, eusocial Hymenoptera that make cooperation a driving force of their adaptations over time and have a unique system of inheritance involving haplodiploidy that allows the colony to function as an individual while only the queen reproduces.

Wilson and Sober's work revived interest in multilevel selection. In a 2005 article, E. O. Wilson argued that kin selection could no longer be thought of as underlying the evolution of extreme sociality, for two reasons. First, he suggested, the argument that haplodiploid inheritance (as in the Hymenoptera) creates a strong selection pressure towards nonreproductive castes is mathematically flawed. Second, eusociality no longer seems to be confined to the hymenopterans; increasing numbers of highly social taxa have been found in the years since Wilson's foundational text Sociobiology: A New Synthesis was published in 1975. These including a variety of insect species, as well as two rodent species (the naked mole-rat and the Damaraland mole rat). Wilson suggests the equation for Hamilton's rule:

rb > c

(where b represents the benefit to the recipient of altruism, c the cost to the altruist, and r their degree of relatedness) should be replaced by the more general equation

rbk + be > c

in which bk is the benefit to kin (b in the original equation) and be is the benefit accruing to the group as a whole. He then argues that, in the present state of the evidence in relation to social insects, it appears that be>rbk, so that altruism needs to be explained in terms of selection at the colony level rather than at the kin level. However, kin selection and group selection are not distinct processes, and the effects of multi-level selection are already accounted for in Hamilton's rule, rb>c, provided that an expanded definition of r, not requiring Hamilton's original assumption of direct genealogical relatedness, is used, as proposed by E. O. Wilson himself.

Spatial populations of predators and prey show restraint of reproduction at equilibrium, both individually and through social communication, as originally proposed by Wynne-Edwards. While these spatial populations do not have well-defined groups for group selection, the local spatial interactions of organisms in transient groups are sufficient to lead to a kind of multi-level selection. There is however as yet no evidence that these processes operate in the situations where Wynne-Edwards posited them.

Rauch et al.'s analysis of host-parasite evolution is broadly hostile to group selection. Specifically, the parasites do not individually moderate their transmission; rather, more transmissible variants – which have a short-term but unsustainable advantage – arise, increase, and go extinct.

Applications

Differing evolutionarily stable strategies

The problem with group selection is that for a whole group to get a single trait, it must spread through the whole group first by regular evolution. But, as J. L. Mackie suggested, when there are many different groups, each with a different evolutionarily stable strategy, there is selection between the different strategies, since some are worse than others. For example, a group where altruism was universal would indeed outcompete a group where every creature acted in its own interest, so group selection might seem feasible; but a mixed group of altruists and non-altruists would be vulnerable to cheating by non-altruists within the group, so group selection would collapse.

Implications in population biology

Social behaviors such as altruism and group relationships can impact many aspects of population dynamics, such as intraspecific competition and interspecific interactions. In 1871, Darwin argued that group selection occurs when the benefits of cooperation or altruism between subpopulations are greater than the individual benefits of egotism within a subpopulation. This supports the idea of multilevel selection, but kinship also plays an integral role because many subpopulations are composed of closely related individuals. An example of this can be found in lions, which are simultaneously cooperative and territorial. Within a pride, males protect the pride from outside males, and females, who are commonly sisters, communally raise cubs and hunt. However, this cooperation seems to be density dependent. When resources are limited, group selection favors prides that work together to hunt. When prey is abundant, cooperation is no longer beneficial enough to outweigh the disadvantages of altruism, and hunting is no longer cooperative.

Interactions between different species can also be affected by multilevel selection. Predator-prey relationships can also be affected. Individuals of certain monkey species howl to warn the group of the approach of a predator. The evolution of this trait benefits the group by providing protection, but could be disadvantageous to the individual if the howling draws the predator's attention to them. By affecting these interspecific interactions, multilevel and kinship selection can change the population dynamics of an ecosystem.

Multilevel selection attempts to explain the evolution of altruistic behavior in terms of quantitative genetics. Increased frequency or fixation of altruistic alleles can be accomplished through kin selection, in which individuals engage in altruistic behavior to promote the fitness of genetically similar individuals such as siblings. However, this can lead to inbreeding depression, which typically lowers the overall fitness of a population. However, if altruism were to be selected for through an emphasis on benefit to the group as opposed to relatedness and benefit to kin, both the altruistic trait and genetic diversity could be preserved. However, relatedness should still remain a key consideration in studies of multilevel selection. Experimentally imposed multilevel selection on Japanese quail was more effective by an order of magnitude on closely related kin groups than on randomized groups of individuals.

Gene-culture coevolution in humans

Gene-culture coevolution allows humans to develop complex artefacts like elaborately decorated temples
Humanity has developed extremely rapidly, arguably through gene-culture coevolution, leading to complex cultural artefacts like the gopuram of the Sri Mariammam temple, Singapore.

Gene-culture coevolution (also called dual inheritance theory) is a modern hypothesis (applicable mostly to humans) that combines evolutionary biology and modern sociobiology to indicate group selection. It is believed that this approach of combining genetic influence with cultural influence over several generations is not present in the other hypotheses such as reciprocal altruism and kin selection, making gene-culture evolution one of the strongest realistic hypotheses for group selection. Fehr provides evidence of group selection taking place in humans presently with experimentation through logic games such as prisoner's dilemma, the type of thinking that humans have developed many generations ago.

Gene-culture coevolution allows humans to develop highly distinct adaptations to the local pressures and environments more quickly than with genetic evolution alone. Robert Boyd and Peter J. Richerson, two strong proponents of cultural evolution, postulate that the act of social learning, or learning in a group as done in group selection, allows human populations to accrue information over many generations. This leads to cultural evolution of behaviors and technology alongside genetic evolution. Boyd and Richerson believe that the ability to collaborate evolved during the Middle Pleistocene, a million years ago, in response to a rapidly changing climate.

In 2003, the behavioral scientist Herbert Gintis examined cultural evolution statistically, offering evidence that societies that promote pro-social norms have higher survival rates than societies that do not. Gintis wrote that genetic and cultural evolution can work together. Genes transfer information in DNA, and cultures transfer information encoded in brains, artifacts, or documents. Language, tools, lethal weapons, fire, cooking, etc., have a long-term effect on genetics. For example, cooking led to a reduction of size of the human gut, since less digestion is needed for cooked food. Language led to a change in the human larynx and an increase in brain size. Projectile weapons led to changes in human hands and shoulders, such that humans are much better at throwing objects than the closest human relative, the chimpanzee.

In 2015, William Yaworsky and colleagues surveyed the opinions of anthropologists on group selection, finding that these varied with the gender and politics of the social scientists concerned. In 2019, Howard Rachlin and colleagues proposed group selection of behavioural patterns, such as learned altruism, during ontogeny parallel to group selection during phylogeny.

Criticism

The use of the Price equation to support group selection was challenged by van Veelen in 2012, arguing that it is based on invalid mathematical assumptions.

Advocates of the gene-centered view of evolution such as Dawkins and Daniel Dennett remain unconvinced about group selection. Dawkins suggests that group selection fails to make an appropriate distinction between replicators and vehicles. The evolutionary biologist Jerry Coyne summarizes the arguments in The New York Review of Books in non-technical terms as follows:

Group selection isn't widely accepted by evolutionists for several reasons. First, it's not an efficient way to select for traits, like altruistic behavior, that are supposed to be detrimental to the individual but good for the group. Groups divide to form other groups much less often than organisms reproduce to form other organisms, so group selection for altruism would be unlikely to override the tendency of each group to quickly lose its altruists through natural selection favoring cheaters. Further, little evidence exists that selection on groups has promoted the evolution of any trait. Finally, other, more plausible evolutionary forces, like direct selection on individuals for reciprocal support, could have made humans prosocial. These reasons explain why only a few biologists, like [David Sloan] Wilson and E. O. Wilson (no relation), advocate group selection as the evolutionary source of cooperation.

The psychologist Steven Pinker states that "group selection has no useful role to play in psychology or social science", since in these domains it "is not a precise implementation of the theory of natural selection, as it is, say, in genetic algorithms or artificial life simulations. Instead [in psychology] it is a loose metaphor, more like the struggle among kinds of tires or telephones."

Saturday, May 17, 2025

Kin selection

From Wikipedia, the free encyclopedia
The co-operative behaviour of social insects like the honey bee can be explained by kin selection.

Kin selection is a process whereby natural selection favours a trait due to its positive effects on the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin selection can lead to the evolution of altruistic behaviour. It is related to inclusive fitness, which combines the number of offspring produced with the number an individual can ensure the production of by supporting others (weighted by the relatedness between individuals). A broader definition of kin selection includes selection acting on interactions between individuals who share a gene of interest even if the gene is not shared due to common ancestry.

Charles Darwin discussed the concept of kin selection in his 1859 book, On the Origin of Species, where he reflected on the puzzle of sterile social insects, such as honey bees, which leave reproduction to their mothers, arguing that a selection benefit to related organisms (the same "stock") would allow the evolution of a trait that confers the benefit but destroys an individual at the same time. J.B.S. Haldane in 1955 briefly alluded to the principle in limited circumstances (Haldane famously joked that he would willingly die for two brothers or eight cousins), and R.A. Fisher mentioned a similar principle even more briefly in 1930. However, it was not until 1964 that W.D. Hamilton generalised the concept and developed it mathematically (resulting in Hamilton's rule) that it began to be widely accepted. The mathematical treatment was made more elegant in 1970 due to advances made by George R. Price. The term "kin selection" was first used by John Maynard Smith in 1964.

According to Hamilton's rule, kin selection causes genes to increase in frequency when the genetic relatedness of a recipient to an actor multiplied by the benefit to the recipient is greater than the reproductive cost to the actor. Hamilton proposed two mechanisms for kin selection. First, kin recognition allows individuals to be able to identify their relatives. Second, in viscous populations, populations in which the movement of organisms from their place of birth is relatively slow, local interactions tend to be among relatives by default. The viscous population mechanism makes kin selection and social cooperation possible in the absence of kin recognition. In this case, nurture kinship, the interaction between related individuals, simply as a result of living in each other's proximity, is sufficient for kin selection, given reasonable assumptions about population dispersal rates. Kin selection is not the same thing as group selection, where natural selection is believed to act on the group as a whole.

In humans, altruism is both more likely and on a larger scale with kin than with unrelated individuals; for example, humans give presents according to how closely related they are to the recipient. In other species, vervet monkeys use allomothering, where related females such as older sisters or grandmothers often care for young, according to their relatedness. The social shrimp Synalpheus regalis protects juveniles within highly related colonies.

Historical overview

Charles Darwin wrote that selection could be applied to the family as well as to the individual.

Charles Darwin was the first to discuss the concept of kin selection (without using that term). In On the Origin of Species, he wrote about the conundrum represented by altruistic sterile social insects that:

This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Breeders of cattle wish the flesh and fat to be well marbled together. An animal thus characterised has been slaughtered, but the breeder has gone with confidence to the same stock and has succeeded.

— Darwin

In this passage "the family" and "stock" stand for a kin group. These passages and others by Darwin about kin selection are highlighted in D.J. Futuyma's textbook of reference Evolutionary Biology and in E. O. Wilson's Sociobiology.

Kin selection was briefly referred to by R.A. Fisher in 1930 and J.B.S. Haldane in 1932 and 1955. J.B.S. Haldane grasped the basic quantities in kin selection, famously writing "I would lay down my life for two brothers or eight cousins". Haldane's remark alluded to the fact that if an individual loses its life to save two siblings, four nephews, or eight cousins, it is a "fair deal" in evolutionary terms, as siblings are on average 50% identical by descent, nephews 25%, and cousins 12.5% (in a diploid population that is randomly mating and previously outbred). But Haldane also joked that he would truly die only to save more than a single identical twin of his or more than two full siblings. In 1955 he clarified:

Let us suppose that you carry a rare gene that affects your behaviour so that you jump into a flooded river and save a child, but you have one chance in ten of being drowned, while I do not possess the gene, and stand on the bank and watch the child drown. If the child's your own child or your brother or sister, there is an even chance that this child will also have this gene, so five genes will be saved in children for one lost in an adult. If you save a grandchild or a nephew, the advantage is only two and a half to one. If you only save a first cousin, the effect is very slight. If you try to save your first cousin once removed the population is more likely to lose this valuable gene than to gain it. … It is clear that genes making for conduct of this kind would only have a chance of spreading in rather small populations when most of the children were fairly near relatives of the man who risked his life.

W. D. Hamilton, in 1963 and especially in 1964 generalised the concept and developed it mathematically, showing that it holds for genes even when they are not rare, deriving Hamilton's rule and defining a new quantity known as an individual's inclusive fitness. He is widely credited as the founder of the field of social evolution. A more elegant mathematical treatment was made possible by George Price in 1970.

The evolutionary biologist John Maynard Smith used the term "kin selection" in 1964.

John Maynard Smith may have coined the actual term "kin selection" in 1964:

These processes I will call kin selection and group selection respectively. Kin selection has been discussed by Haldane and by Hamilton. … By kin selection I mean the evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which do not require any discontinuities in the population breeding structure.

Kin selection causes changes in gene frequency across generations, driven by interactions between related individuals. This dynamic forms the conceptual basis of the theory of sociobiology. Some cases of evolution by natural selection can only be understood by considering how biological relatives influence each other's fitness. Under natural selection, a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population; and conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a hypothetical gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behaviour, may nonetheless increase in frequency, because relatives often carry the same gene. According to this principle, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour, making kin selection possible. This is a special case of a more general model, "inclusive fitness". This analysis has been challenged, Wilson writing that "the foundations of the general theory of inclusive fitness based on the theory of kin selection have crumbled" and that he now relies instead on the theory of eusociality and "gene-culture co-evolution" for the underlying mechanics of sociobiology. Inclusive fitness theory is still generally accepted however, as demonstrated by the publication of a rebuttal to Wilson's claims in Nature from over a hundred researchers.

Kin selection is contrasted with group selection, according to which a genetic trait can become prevalent within a group because it benefits the group as a whole, regardless of any benefit to individual organisms. All known forms of group selection conform to the principle that an individual behaviour can be evolutionarily successful only if the genes responsible for this behaviour conform to Hamilton's Rule, and hence, on balance and in the aggregate, benefit from the behaviour.

Hamilton's rule

Formally, genes should increase in frequency when

where

r = the genetic relatedness of the recipient to the actor, often defined as the probability that a gene picked randomly from each at the same locus is identical by descent.
B = the additional reproductive benefit gained by the recipient of the altruistic act,
C = the reproductive cost to the individual performing the act.

This inequality is known as Hamilton's rule after W. D. Hamilton who in 1964 published the first formal quantitative treatment of kin selection.

The relatedness parameter (r) in Hamilton's rule was introduced in 1922 by Sewall Wright as a coefficient of relationship that gives the probability that at a random locus, the alleles there will be identical by descent. Modern formulations of the rule use Alan Grafen's definition of relatedness based on the theory of linear regression.

A 2014 review of many lines of evidence for Hamilton's rule found that its predictions were confirmed in a wide variety of social behaviours across a broad phylogenetic range of birds, mammals and insects, in each case comparing social and non-social taxa. Among the experimental findings, a 2010 study used a wild population of red squirrels in Yukon, Canada. Surrogate mothers adopted related orphaned squirrel pups but not unrelated orphans. The cost of adoption was calculated by measuring a decrease in the survival probability of the entire litter after increasing the litter by one pup, while benefit was measured as the increased chance of survival of the orphan. The degree of relatedness of the orphan and surrogate mother for adoption to occur depended on the number of pups the surrogate mother already had in her nest, as this affected the cost of adoption. Females always adopted orphans when rB was greater than C, but never adopted when rB was less than C, supporting Hamilton's rule.

Mechanisms

Altruism occurs where the instigating individual suffers a fitness loss while the receiving individual experiences a fitness gain. The sacrifice of one individual to help another is an example.

Hamilton outlined two ways in which kin selection altruism could be favoured:

The selective advantage which makes behaviour conditional in the right sense on the discrimination of factors which correlate with the relationship of the individual concerned is therefore obvious. It may be, for instance, that in respect of a certain social action performed towards neighbours indiscriminately, an individual is only just breaking even in terms of inclusive fitness. If he could learn to recognise those of his neighbours who really were close relatives and could devote his beneficial actions to them alone an advantage to inclusive fitness would at once appear. Thus a mutation causing such discriminatory behaviour itself benefits inclusive fitness and would be selected. In fact, the individual may not need to perform any discrimination so sophisticated as we suggest here; a difference in the generosity of his behaviour according to whether the situations evoking it were encountered near to, or far from, his own home might occasion an advantage of a similar kind.

Kin recognition and the green beard effect

Kin recognition theory predicts a selective advantage for the bearers of a trait (like the fictitious 'green beard') behave altruistically towards others with the same trait.

First, if individuals have the capacity to recognise kin and to discriminate (positively) on the basis of kinship, then the average relatedness of the recipients of altruism could be high enough for kin selection. Because of the facultative nature of this mechanism, kin recognition and discrimination were expected to be unimportant except among 'higher' forms of life. However, as molecular recognition mechanisms have been shown to operate in organisms such as slime moulds  kin recognition has much wider importance than previously recognised. Kin recognition may be selected for inbreeding avoidance, and little evidence indicates that 'innate' kin recognition plays a role in mediating altruism. A thought experiment on the kin recognition/discrimination distinction is the hypothetical 'green beard', where a gene for social behaviour is imagined also to cause a distinctive phenotype that can be recognised by other carriers of the gene. Due to conflicting genetic similarity in the rest of the genome, there should be selection pressure for green-beard altruistic sacrifices to be suppressed, making common ancestry the most likely form of inclusive fitness. This suppression is overcome if new phenotypes -other beard colours- are formed through mutation or introduced into the population from time to time. This proposed mechanism goes by the name of 'beard chromodynamics'.

Viscous populations

Secondly, indiscriminate altruism may be favoured in "viscous" populations, those with low rates or short ranges of dispersal. Here, social partners are typically related, and so altruism can be selective advantageous without the need for kin recognition and kin discrimination faculties—spatial proximity, together with limited dispersal, ensures that social interactions are more often with related individuals. This suggests a rather general explanation for altruism. Directional selection always favours those with higher rates of fecundity within a certain population. Social individuals can often enhance the survival of their own kin by participating in and following the rules of their own group.

Hamilton later modified his thinking to suggest that an innate ability to recognise actual genetic relatedness was unlikely to be the dominant mediating mechanism for kin altruism:

But once again, we do not expect anything describable as an innate kin recognition adaptation, used for social behaviour other than mating, for the reasons already given in the hypothetical case of the trees.

Hamilton's later clarifications often go unnoticed. Stuart West and colleagues have countered the long-standing assumption that kin selection requires innate powers of kin recognition. Another doubtful assumption is that social cooperation must be based on limited dispersal and shared developmental context. Such ideas have obscured the progress made in applying kin selection to species including humans, on the basis of cue-based mediation of social bonding and social behaviours.

Special cases

Eusociality

Ants are eusocial insects; the queen (large, centre) is reproductive, while the workers (small) and soldiers (medium size, with large jaws) are generally not.

Eusociality (true sociality) occurs in social systems with three characteristics: an overlap in generations between parents and their offspring, cooperative brood care, and specialised castes of non-reproductive individuals. The social insects provide good examples of organisms with what appear to be kin selected traits. The workers of some species are sterile, a trait that would not occur if individual selection was the only process at work. The relatedness coefficient r is abnormally high between the worker sisters in a colony of Hymenoptera due to haplodiploidy. Hamilton's rule is presumed to be satisfied because the benefits in fitness for the workers are believed to exceed the costs in terms of lost reproductive opportunity, though this has never been demonstrated empirically. Competing hypotheses have been offered to explain the evolution of social behaviour in such organisms.

The eusocial shrimp Synalpheus regalis protects juveniles in the colony. By defending the young, the large defender shrimp can increase its inclusive fitness. Allozyme data demonstrated high relatedness within colonies, averaging 0.50. This means that colonies represent close kin groups, supporting the hypothesis of kin selection.

Allomothering

Vervet monkeys behave in ways that imply kin selection.

Vervet monkeys utilise allomothering, parenting by group members other than the actual mother or father, where the allomother is typically an older female sibling or a grandmother. Individuals act aggressively toward other individuals that were aggressive toward their relatives. The behaviour implies kin selection between siblings, between mothers and offspring, and between grandparents and grandchildren.

In humans

Whether or not Hamilton's rule always applies, relatedness is often important for human altruism, in that humans are inclined to behave more altruistically toward kin than toward unrelated individuals. Many people choose to live near relatives, exchange sizeable gifts with relatives, and favour relatives in wills in proportion to their relatedness.

Experimental studies, interviews, and surveys

Interviews of several hundred women in Los Angeles showed that while non-kin friends were willing to help one another, their assistance was far more likely to be reciprocal. The largest amounts of non-reciprocal help, however, were reportedly provided by kin. Additionally, more closely related kin were considered more likely sources of assistance than distant kin. Similarly, several surveys of American college students found that individuals were more likely to incur the cost of assisting kin when a high probability that relatedness and benefit would be greater than cost existed. Participants' feelings of helpfulness were stronger toward family members than non-kin. Additionally, participants were found to be most willing to help those individuals most closely related to them. Interpersonal relationships between kin in general were more supportive and less Machiavellian than those between non-kin.

In one experiment, the longer participants (from both the UK and the South African Zulus) held a painful skiing position, the more money or food was presented to a given relative. Participants repeated the experiment for individuals of different relatedness (parents and siblings at r=.5, grandparents, nieces, and nephews at r=.25, etc.). The results showed that participants held the position for longer intervals the greater the degree of relatedness between themselves and those receiving the reward.

Observational studies

A study of food-sharing practices on the West Caroline islets of Ifaluk determined that food-sharing was more common among people from the same islet, possibly because the degree of relatedness between inhabitants of the same islet would be higher than relatedness between inhabitants of different islets. When food was shared between islets, the distance the sharer was required to travel correlated with the relatedness of the recipient—a greater distance meant that the recipient needed to be a closer relative. The relatedness of the individual and the potential inclusive fitness benefit needed to outweigh the energy cost of transporting the food over distance.

Humans may use the inheritance of material goods and wealth to maximise their inclusive fitness. By providing close kin with inherited wealth, an individual may improve his or her kin's reproductive opportunities and thus increase his or her own inclusive fitness even after death. A study of a thousand wills found that the beneficiaries who received the most inheritance were generally those most closely related to the will's writer. Distant kin received proportionally less inheritance, with the least amount of inheritance going to non-kin.

A study of childcare practices among Canadian women found that respondents with children provide childcare reciprocally with non-kin. The cost of caring for non-kin was balanced by the benefit a woman received—having her own offspring cared for in return. However, respondents without children were significantly more likely to offer childcare to kin. For individuals without their own offspring, the inclusive fitness benefits of providing care to closely related children might outweigh the time and energy costs of childcare.

Family investment in offspring among black South African households also appears consistent with an inclusive fitness model. A higher degree of relatedness between children and their caregivers was correlated with a higher degree of investment in the children, with more food, health care, and clothing. Relatedness was also associated with the regularity of a child's visits to local medical practitioners and with the highest grade the child had completed in school, and negatively associated with children being behind in school for their age.

Observation of the Dolgan hunter-gatherers of northern Russia suggested that there are larger and more frequent asymmetrical transfers of food to kin. Kin are more likely to be welcomed to non-reciprocal meals, while non-kin are discouraged from attending. Finally, when reciprocal food-sharing occurs between families, these families are often closely related, and the primary beneficiaries are the offspring.

Violence in families is more likely when step-parents are present, and that "genetic relationship is associated with a softening of conflict, and people's evident valuations of themselves and of others are systematically related to the parties' reproductive values". Numerous studies suggest how inclusive fitness may work amongst different peoples, such as the Ye'kwana of southern Venezuela, the Gypsies of Hungary, and the doomed Donner Party of the United States.

Human social patterns

Families are important in human behaviour, but kin selection may be based on closeness and other cues.

Evolutionary psychologists, following early human sociobiologists' interpretation of kin selection theory initially attempted to explain human altruistic behaviour through kin selection by stating that "behaviors that help a genetic relative are favored by natural selection." However, many evolutionary psychologists recognise that this common shorthand formulation is inaccurate:

Many misunderstandings persist. In many cases, they result from conflating "coefficient of relatedness" and "proportion of shared genes", which is a short step from the intuitively appealing—but incorrect—interpretation that "animals tend to be altruistic toward those with whom they share a lot of genes." These misunderstandings don't just crop up occasionally; they are repeated in many writings, including undergraduate psychology textbooks—most of them in the field of social psychology, within sections describing evolutionary approaches to altruism.

As with the earlier sociobiological forays into the cross-cultural data, typical approaches are not able to find explanatory fit with the findings of ethnographers insofar that human kinship patterns are not necessarily built upon blood-ties. However, as Hamilton's later refinements of his theory make clear, it does not simply predict that genetically related individuals will inevitably recognise and engage in positive social behaviours with genetic relatives: rather, indirect context-based mechanisms may have evolved, which in historical environments have met the inclusive fitness criterion. Consideration of the demographics of the typical evolutionary environment of any species is crucial to understanding the evolution of social behaviours. As Hamilton himself put it, "Altruistic or selfish acts are only possible when a suitable social object is available. In this sense behaviours are conditional from the start".

Under this perspective, and noting the necessity of a reliable context of interaction being available, the data on how altruism is mediated in social mammals is readily made sense of. In social mammals, primates and humans, altruistic acts that meet the kin selection criterion are typically mediated by circumstantial cues such as shared developmental environment, familiarity and social bonding. That is, it is the context that mediates the development of the bonding process and the expression of the altruistic behaviours, not genetic relatedness as such. This interpretation is compatible with the cross-cultural ethnographic data and has been called nurture kinship.

In plants

Observations

Though originally thought unique to the animal kingdom, evidence of kin selection has been identified in the plant kingdom.

Competition for resources between developing zygotes in plant ovaries increases when seeds had been pollinated with male gametes from different plants. How developing zygotes differentiate between full siblings and half-siblings in the ovary is undetermined, but genetic interactions are thought to play a role. Nonetheless, competition between zygotes in the ovary is detrimental to the reproductive success of the (female) plant, and fewer zygotes mature into seeds. As such, the reproductive traits and behaviors of plants suggests the evolution of behaviors and characteristics that increase the genetic relatedness of fertilized eggs in the plant ovary, thereby fostering kin selection and cooperation among the seeds as they develop. These traits differ among plant species. Some species have evolved to have fewer ovules per ovary, commonly one ovule per ovary, thereby decreasing the chance of developing multiple, differently fathered seeds within the same ovary. Multi-ovulated plants have developed mechanisms that increase the chances of all ovules within the ovary being fathered by the same parent. Such mechanisms include dispersal of pollen in aggregated packets and closure of the stigmatic lobes after pollen is introduced. The aggregated pollen packet releases pollen gametes in the ovary, thereby increasing likelihood that all ovules are fertilized by pollen from the same parent. Likewise, the closure of the ovary pore prevents entry of new pollen. Other multi-ovulated plants have evolved mechanisms that mimic the evolutionary adaption of single-ovulated ovaries; the ovules are fertilized by pollen from different individuals, but the mother ovary then selectively aborts fertilized ovules, either at the zygotic or embryonic stage.

Morning glory plants grow smaller roots when next to kin than to non-kin plants.

After seeds are dispersed, kin recognition and cooperation affects root formation in developing plants. Studies have found that the total root mass developed by Ipomoea hederacea (morning glory shrubs) grown next to kin is significantly smaller than those grown next to non-kin; shrubs grown next to kin thus allocate less energy and resources to growing the larger root systems needed for competitive growth. When seedlings were grown in individual pots placed next to kin or non-kin relatives, no difference in root growth was observed. This indicates that kin recognition occurs via signals received by the roots. Further, groups of I. hederacea plants are more varied in height when grown with kin than when grown with non-kin. The evolutionary benefit provided by this was further investigated by researchers at the Université de Montpellier. They found that the alternating heights seen in kin-grouped crops allowed for optimal light availability to all plants in the group; shorter plants next to taller plants had access to more light than those surrounded by plants of similar height.

The above examples illustrate the effect of kin selection in the equitable allocation of light, nutrients, and water. The evolutionary emergence of single-ovulated ovaries in plants has eliminated the need for a developing seed to compete for nutrients, thus increasing its chance of survival and germination. Likewise, the fathering of all ovules in multi-ovulated ovaries by one father, decreases the likelihood of competition between developing seeds, thereby also increasing the seeds' chances of survival and germination. The decreased root growth in plants grown with kin increases the amount of energy available for reproduction; plants grown with kin produced more seeds than those grown with non-kin. Similarly, the increase in light made available by alternating heights in groups of related plants is associated with higher fecundity.

Kin selection has also been observed in plant responses to herbivory. In an experiment done by Richard Karban et al., leaves of potted Artemisia tridentata (sagebrushes) were clipped with scissors to simulate herbivory. The gaseous volatiles emitted by the clipped leaves were captured in a plastic bag. When these volatiles were transferred to leaves of a closely related sagebrush, the recipient experienced lower levels of herbivory than those that had been exposed to volatiles released by non-kin plants. Sagebrushes do not uniformly emit the same volatiles in response to herbivory: the chemical ratios and composition of emitted volatiles vary from one sagebrush to another. Closely related sagebrushes emit similar volatiles, and the similarities decrease as relatedness decreases. This suggests that the composition of volatile gasses plays a role in kin selection among plants. Volatiles from a distantly related plant are less likely to induce a protective response against herbivory in a neighboring plant, than volatiles from a closely related plant. This fosters kin selection, as the volatiles emitted by a plant will activate the herbivorous defense response in related plants only, thus increasing their chance of survival and reproduction.

Kin selection may play a role in plant-pollinator interactions, especially because pollinator attraction is influenced not only by floral displays, but by the spatial arrangement of plants in a group, which is referred to as the "magnet effect". For example, in an experiment performed on Moricandia moricandioides, Torices et al. demonstrated that focal plants in the presence of kin show increased advertising effort (defined as total petal mass of plants in a group divided by the plant biomass) compared to those in the presence of non-kin, and that this effect is greater in larger groups. M. moricandioides is a good model organism for the study of plant-pollinator interactions because it relies on pollinators for reproduction, as it is self-incompatible. The study design for this experiment included planting establishing pots of M. moricandioides with zero, three or six neighbors (either unrelated or half-sib progeny of the same mother) and advertising effort was calculated after 26 days of flowering. The exact mechanism of kin recognition in M. moricandioides is unknown, but possible mechanisms include above-ground communication with volatile compounds, or below-ground communication with root exudates.

Mechanisms in plants

The ability to differentiate between kin and non-kin is not necessary for kin selection in many animals. However, because plants do not reliably germinate in close proximity to kin, it is thought that, within the plant kingdom, kin recognition is especially important for kin selection there, but the mechanism remains unknown.

One proposed mechanism for kin recognition involves communication through roots, with secretion and reception of root exudates. This would require exudates to be actively secreted by roots of one plant, and detected by roots of neighboring plants. The root exudate allantoin produced by rice plants, Oryza sativa, has been documented to be in greater production when growing next to cultivars that are largely unrelated. High production levels of Allantoin correlated to up regulation of auxin and auxin transporters, resulting in increased lateral root development and directional growth of their roots towards non kin, maximizing competition. This is mainly not observed in Oryza Sativa when surrounded by kin, invoking altruistic behaviors to promote inclusive fitness. However the root receptors responsible for recognition of kin exudates, and the pathway induced by receptor activation, remain unknown.[68] The mycorrhiza associated with roots might facilitate reception of exudates, but again the mechanism is unknown.

Another possibility is communication through green leaf volatiles. Karban et al. studied kin recognition in sagebrushes, Artemisia tridentata. The volatile-donating sagebrushes were kept in individual pots, separate from the plants that received the volatiles, finding that plants responded to herbivore damage to a neighbour's leaves. This suggests that root signalling is not necessary to induce a protective response against herbivory in neighbouring kin plants. Karban et al. suggest that plants may be able to differentiate between kin and non-kin based on the composition of volatiles. Because only the recipient sagebrush's leaves were exposed the volatiles presumably activated a receptor protein in the plant's leaves. The identity of this receptor, and the signalling pathway triggered by its activation, both remain to be discovered.

Objections

The theory of kin selection has been criticised by W. J. Alonso (in 1998) and by Alonso and C. Schuck-Paim (in 2002). They argue that the behaviours which kin selection attempts to explain are not altruistic (in pure Darwinian terms) because: (1) they may directly favour the performer as an individual aiming to maximise its progeny (so the behaviours can be explained as ordinary individual selection); (2) these behaviours benefit the group (so they can be explained as group selection); or (3) they are by-products of a developmental system of many "individuals" performing different tasks (like a colony of bees, or the cells of multicellular organisms, which are the focus of selection). They also argue that the genes involved in sex ratio conflicts could be treated as "parasites" of (already established) social colonies, not as their "promoters", and, therefore the sex ratio in colonies would be irrelevant to the transition to eusociality. Those ideas were mostly ignored until they were put forward again in a series of controversial papers by E. O. Wilson, Bert Hölldobler, Martin Nowak and Corina Tarnita. Nowak, Tarnita and Wilson argued that

Inclusive fitness theory is not a simplification over the standard approach. It is an alternative accounting method, but one that works only in a very limited domain. Whenever inclusive fitness does work, the results are identical to those of the standard approach. Inclusive fitness theory is an unnecessary detour, which does not provide additional insight or information.

— Nowak, Tarnita, and Wilson

They, like Alonso and Schuck-Paim, argue for a multi-level selection model instead. This aroused a strong response, including a rebuttal published in Nature from over a hundred researchers.

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