Human mating strategies

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People seek out a mate for an intimate relationship

 

In evolutionary psychology and behavioral ecology, human mating strategies are a set of behaviors used by individuals to attract, select, and retain mates. Mating strategies overlap with reproductive strategies, which encompass a broader set of behaviors involving the timing of reproduction and the trade-off between quantity and quality of offspring. 

Relative to other animals, human mating strategies are unique in their relationship with cultural variables such as the institution of marriage. Humans may seek out individuals with the intention of forming a long-term intimate relationship, marriage, casual relationship, or friendship. The human desire for companionship is one of the strongest human drives. It is an innate feature of human nature, and may be related to the sex drive. The human mating process encompasses the social and cultural processes whereby one person may meet another to assess suitability, the courtship process and the process of forming an interpersonal relationship. Commonalities, however, can be found between humans and nonhuman animals in mating behavior.

Flirting

To bond or express sexual interest, people flirt. Kate Fox, a social anthropologist, posits two main types of flirting: flirting for fun and flirting with intent. Flirting for fun can take place between friends, co-workers, or total strangers that wish to get to know each other. This type of flirting does not seek sexual intercourse or romantic relationship, but increases the bonds between two people. 

Flirting with intent plays a role in mate-selection. The person flirting sends out signals of sexual availability to another, and hopes to see the interest returned to encourage continued flirting. Flirting can involve non-verbal signs, such as an exchange of glances, hand-touching, hair-touching, or verbal signs, such as chatting up, flattering comments, and exchange of telephone numbers to enable further contact.

Dating

People date to assess each other's suitability as a partner in an intimate relationship or as a spouse. Dating rules may vary across different cultures, and some societies may even replace the dating process by a courtship instead.

Matchmaking

In many cultural traditions, a date may be arranged by a third party, who may be a family member, acquaintance, or professional matchmaker. In some cultures, a marriage may be arranged by the couple's parents or an outside party. Recently, internet dating has become popular.

Theoretical background

Research on human mating strategies is guided by the theory of sexual selection, and in particular, Robert Trivers' concept of parental investment. Trivers defines parental investment as “any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.” Trivers posited that differential parental investment between males and females drives the process of sexual selection, which leads to the evolution of sexual dimorphism in mate choice, competitive ability, and courtship displays (see also secondary sex characteristics). In humans, females make a larger parental investment than males (i.e. nine months of gestation followed by childbirth and lactation). While human males invest heavily in their offspring as well, their minimum parental investment is still lower than that of females. Hence, evolutionary psychologists have predicted a number of sex differences in human mating strategies.

Gender differences

Sexual desire

One theory states that because of their lower minimum parental investment, men can achieve greater reproductive success by mating with multiple women than women can achieve by mating with multiple men. Evolutionary psychologists therefore argue that ancestral men who possessed a desire for multiple short-term sex partners, to the extent that they were capable of attracting them, would have left more descendants than men without such a desire. Ancestral women, by contrast, would have maximized reproductive success not by mating with as many men as possible, but by selectively mating with those men who were most able and willing to invest resources in their offspring. Gradually in a bid to compete to get resources from potential men, women have evolved to show extended sexuality. One classic study found that when college students were approached on campus by opposite-sex confederates and asked if they wanted to "go to bed" with him/her, 75% of the men said yes while 0% percent of the women said yes. Evidence also indicates that, across cultures, men report a greater openness to casual sex, a larger desired number of sexual partners, and a greater desire to have sex sooner in a relationship. These sex differences have been shown to be reliable across various studies and methodologies. However, there is some controversy as to the scope and interpretation of these sex differences.

Evolutionary research often indicates that men have a strong desire for casual sex, unlike women. Men are often depicted as wanting numerous female sexual partners to maximize reproductive success. Evolutionary mechanisms for short-term mating are evident today. Mate-guarding behavior and sexual jealousy point to an evolutionary history in which sexual relations with multiple partners became a recurrent adaptive problem, while the willingness of modern-day men to have sex with attractive strangers, and the prevalence of extramarital affairs in similar frequencies cross-culturally, are evidence of an ancestral past in which polygamous mating strategies were adopted. By contrast, journalist Daniel Bergner, who dismisses evolutionary biology, argues that monogamy has been used to control human female sexual behavior and that the human female sex drive is not lower than the human male sex drive.

Flanagan and Cardwell argue that men could not pursue this ideology without willing female partners. Every time a man has a new sexual partner, the woman also has a new sexual partner. It has been proposed, therefore, that casual sex and numerous sexual partners may also confer some benefit to females. That is, they would produce more genetically diverse offspring as a result, which would increase their chances of successfully rearing children to adolescence, or independence.

Sexual attractions

Evolutionary psychologists have predicted that men generally place a greater value on youth and physical attractiveness in a mate than do women. Youth is associated with reproductive value in women, and features that men find physically attractive in women are thought to signal health and fertility. Men who preferentially mated with healthy, fertile, and reproductively valuable women would have left more descendants than men who did not. Since men’s reproductive value does not decline as steeply with age as does women’s, women are not expected to exhibit as strong of a preference for youth in a mate. Evolutionary psychologists have also speculated that women are relatively more attracted to ambition and social status in a mate because they associate these characteristics with men’s access to resources. Women who preferentially mated with men capable of investing resources in their offspring, thereby ensuring their offsprings' survival, would have left more descendants than women who did not. Evolutionary psychologists have tested these predictions across cultures, confirming that men tend to report a greater preference for youth and physical attractiveness in a mate than do women, and that women tend to report a greater preference for ambition and social status in a mate than do men. Some sex differences in mate preferences may be attenuated by national levels of gender equity and gender empowerment. The specific role that culture plays in modulating sex differences in mate preferences is subject to debate. Cultural variations in mate preference can be due to the evolved differences between males and females of a culture. For example, as women gain more access to resources their mate preferences change. Finding a mate with resources becomes less of a priority and a mate with domestic skills is more important. As women’s access to resources varies between cultures, so does mate preference.

Individual differences

Sociosexual Orientation Inventory

Average differences in mating strategies between the sexes do not entail uniformity in mating strategies within the sexes, and in humans, such within-sex variation is substantial. Individual differences in mating strategies are commonly measured using the Sociosexual Orientation Inventory (SOI), a questionnaire that includes items assessing past sexual behavior, anticipated future sexual behavior, and openness to casual sex. Higher scores on the SOI indicate a sexually unrestricted mating strategy, and lower scores on the SOI indicate a sexually restricted mating strategy. Several studies have found that scores on the SOI are related to mate preferences, with more sexually restricted individuals preferring personal/parenting qualities in a mate (e.g. responsibility and loyalty), and with less sexual restricted individual preferring qualities related to physical attractiveness and social visibility. Other studies have shown that SOI scores are related to personality traits (i.e. extraversion, erotophilia, and low agreeableness), conspicuous consumption in men as a means to attract women, and increased allocation of visual attention to attractive opposite-sex faces.

Short-term vs. long-term mating

Evolutionary psychologists have proposed that individuals may adopt conditional mating strategies in which they adjust their mating tactics to relevant environmental or internal conditions. To the extent that ancestral men were capable of pursuing short-term mating strategies with multiple women, the evolutionary benefits are relatively straightforward. Less clear, however, are the evolutionary benefits that women might have received from pursuing short-term mating strategies. However, women in a stressed situation may benefit from protection from a male and short term mating is a way to achieve this as is seen in contemporary asylum seeker anthropological studies . One prominent hypothesis is that ancestral women selectively engaged in short-term mating with men capable of transmitting genetic benefits to their offspring such as health, disease resistance, or attractiveness. Since women cannot inspect men's genes directly, they may have evolved to infer genetic quality from certain observable characteristics. One prominent candidate for a "good genes" indicator includes fluctuating asymmetry, or the degree to which men deviate from perfect bodily symmetry. Other candidates include masculine facial features, behavioral dominance, and low vocal pitch. Evolutionary psychologists have therefore indicated that women pursuing a short-term mating strategy have higher preferences for these good gene indicators, and men who possess good genes indicators are more successful in pursuing short-term mating strategies than men who do not. Indeed, research indicates that self-perceived physical attractiveness, fluctuating asymmetry, and low vocal pitch are positively related to short-term mating success in men but not in women. Women prefer purported good genes indicators more for a short-term mate than for a long-term mate, and a related line of research, known as the ovulatory shift hypothesis, shows that women’s preferences for good genes indicators in short-term mates tends to increase during peak fertility in the menstrual cycle just prior to ovulation.

Women are thought to seek long-term partners with resources (such as shelter and food) that provide aid and support survival of offspring. To achieve this, women are thought to have evolved extended sexuality.

Mating strategy plasticity

Research on the conditional nature of mating strategies has revealed that long-term and short-term mating preferences can be fairly plastic. Following exposure to cues that would have been affected mating in the ancestral past, both men and women appear to adjust their mating preferences in ways that would have historically enhanced their fitness. Such cues include the need to care for young, danger from animals and other humans, and resource availability.

Environmental predictors

In 2005, the evolutionary psychologist David Schmitt conducted a multinational survey of sexual attitudes and behaviors involving 48 countries called the International Sexual Description Project (ISSR). Schmitt assessed relationships between several societal-level variables and average scores on the SOI. One variable that was shown to significantly predict a nation’s average SOI score was the Operational Sex Ratio (OSR), which was defined by Schmitt as “the relative balance of marriage-age men versus marriage-age women in the local mating pool.” When one sex is scarce relative to the other sex, the less-scarce sex may compete more intensely for access to the scarcer sex. One way in which the more numerous sex might compete is by displaying the attributes that are most desired by the scarcer sex. Since men have a greater desire for casual sex (see above), societies with more women relative to men were predicted to exhibit higher scores on the SOI than societies with more balanced or male-biased sex ratios. This prediction was confirmed: OSR was significantly positively correlated with national SOI scores. Another variable that Schmitt predicted would influence SOI scores was the need for biparental care. In societies where extensive care from both parents is needed to ensure offspring survival, the costs of having sex with an uncommitted partner are much higher. Schmitt found significant negative correlations between several indices of need for biparental care (e.g. infant mortality, child malnutrition, and low birth-weight infants) and national SOI scores. 

Another important societal variable for mating strategies is the threat of infectious disease or pathogen prevalence. Since physical attractiveness is thought to signal health and disease resistance, evolutionary psychologists have predicted that, in societies high in pathogen prevalence, people value attractiveness more in a mate. Indeed, research has confirmed that pathogen prevalence is associated with preferences for attractiveness across nations. Women in nations with high pathogen prevalence also show greater preferences for facial masculinity. Researchers have also reasoned that sexual contact with multiple individuals increases the risk of disease transmission, thereby increasing the costs of pursuing a short-term mating strategy. Consistent with this reasoning, higher pathogen prevalence is associated with lower national SOI scores. Finally, several studies have found that experimentally manipulating disease salience has a causal influence on attractiveness preferences and SOI scores in predicted directions.

Political attitudes

Some evolutionary psychologists have argued that mating strategies can influence political attitudes. According to this perspective, different mating strategies are in direct strategic conflict. For instance, the stability of long-term partnerships may be threatened by the availability of short-term sexual opportunities. Therefore, public policy measures that impose costs on casual sex may benefit people pursuing long-term mating strategies by reducing the availability of short-term mating opportunities outside of committed relationships. One public policy measure that imposes costs on people pursuing short-term mating strategies, and may thereby appeal to sexually restricted individuals, is the banning of abortion. In an influential doctoral dissertation, the psychologist Jason Weeden conducted statistical analyses on public and undergraduate datasets supporting the hypothesis that attitudes towards abortion are more strongly predicted by mating-relevant variables than by variables related to views on the sanctity of life.

Weeden and colleagues have also argued that attitudes towards drug legalization are driven by individual differences in mating strategies. Insofar as sexually restricted individuals associate recreational drug use with promiscuity, they may be motivated to oppose drug legalization. Consistent with this, one study found that the strongest predictor of attitudes towards drug legalization was scores on the SOI. This relationship remained strong even when controlling for personality traits, political orientation, and moral values. By contrast, nonsexual variables typically associated with attitudes towards drug legalization were strongly attenuated or eliminated when controlling for SOI and other sexuality-related measures. These findings were replicated in Belgium, Japan, and the Netherlands. Weeden and colleagues have made similar arguments and have conducted similar analyses in regard to religiosity; that is, religious institutions may function to facilitate high-fertility, sexually restricted mating and reproductive strategies.

Parental investment

From Wikipedia, the free encyclopedia

A female calliope hummingbird feeding her chicks

 

Ronald Fisher

 

A human mother feeding her child

 

Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure (e.g. time, energy, resources) that benefits offspring. Parental investment may be performed by both males and females (biparental care), females alone (exclusive maternal care) or males alone (exclusive paternal care). Care can be provided at any stage of the offspring's life, from pre-natal (e.g. egg guarding and incubation in birds, and placental nourishment in mammals) to post-natal (e.g. food provisioning and protection of offspring). 

Parental investment theory, a term coined by Robert Trivers in 1972, predicts that the sex that invests more in its offspring will be more selective when choosing a mate, and the less-investing sex will have intra-sexual competition for access to mates. This theory has been influential in explaining sex differences in sexual selection and mate preferences, throughout the animal kingdom and in humans.

History

In 1859, Charles Darwin published On the Origin of Species. This introduced the concept of natural selection to the world, as well as related theories such as sexual selection. For the first time, evolutionary theory was used to explain why females are "coy" and males are "ardent" and compete with each other for females' attention. In 1930, Ronald Fisher wrote The Genetical Theory of Natural Selection, in which he introduced the modern concept of parental investment, introduced the sexy son hypothesis, and introduced Fisher's principle. In 1948, Angus John Bateman published an influential study of fruit flies in which he concluded that because female gametes are more costly to produce than male gametes, the reproductive success of females was limited by the ability to produce ovum, and the reproductive success of males was limited by access to females. In 1972, Trivers continued this line of thinking with his proposal of parental investment theory, which describes how parental investment affects sexual behavior. He concludes that the sex that has higher parental investment will be more selective when choosing a mate, and the sex with lower investment will compete intra-sexually for mating opportunities. In 1974, Trivers extended parental investment theory to explain parent-offspring conflict, the conflict between investment that is optimal from the parent's versus the offspring's perspective.

Parental care

Parental investment theory is a branch of life history theory. The earliest consideration of parental investment is given by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection, wherein Fisher argued that parental expenditure on both sexes of offspring should be equal. Clutton-Brock expanded the concept of parental investment to include costs to any other component of parental fitness.

Male dunnocks tend to not discriminate between their own young and those of another male in polyandrous or polygynandrous systems. They increase their own reproductive success through feeding the offspring in relation to their own access to the female throughout the mating period, which is generally a good predictor of paternity. This indiscriminative parental care by males is also observed in redlip blennies.

A cellar spider defending spiderlings.

 

In some insects, male parental investment is given in the form of a nuptial gift. For instance, ornate moth females receive a spermatophore containing nutrients, sperm and defensive toxins from the male during copulation. This gift, which can account for up to 10% of the male's body mass, constitutes the total parental investment the male provides.

In some species, such as humans and many birds, the offspring are altricial and unable to fend for themselves for an extended period of time after birth. In these species, males invest more in their offspring than do the male parents of precocial species, since reproductive success would otherwise suffer. 

 

A female lizard defending her clutch against an egg-eating snake.

The benefits of parental investment to the offspring are large and are associated with the effects on condition, growth, survival, and ultimately on reproductive success of the offspring. For example, in the cichlid fish Tropheus moorii, a female has very high parental investment in her young because she mouthbroods the young and while mouthbrooding, all nourishment she takes in goes to feed the young and she effectively starves herself. In doing this, her young are larger, heavier, and faster than they would have been without it. These benefits are very advantageous since it lowers their risk of being eaten by predators and size is usually the determining factor in conflicts over resources. However, such benefits can come at the cost of parent's ability to reproduce in the future e.g., through increased risk of injury when defending offspring against predators, loss of mating opportunities whilst rearing offspring, and an increase in the time interval until the next reproduction.

A special case of parental investment is when young do need nourishment and protection, but the genetic parents do not actually contribute in the effort to raise their own offspring. For example, in Bombus terrestris, oftentimes sterile female workers will not reproduce on their own, but will raise their mother's brood instead. This is common in social Hymenoptera due to haplodiploidy, whereby males are haploid and females are diploid. This ensures that sisters are more related to each other than they ever would be to their own offspring, incentivizing them to help raise their mother's young over their own.

Overall, parents are selected to maximize the difference between the benefits and the costs, and parental care will be likely to evolve when the benefits exceed the costs.

Parent-offspring conflict

Reproduction is costly. Individuals are limited in the degree to which they can devote time and resources to producing and raising their young, and such expenditure may also be detrimental to their future condition, survival, and further reproductive output. However, such expenditure is typically beneficial to the offspring, enhancing their condition, survival, and reproductive success. These differences may lead to parent-offspring conflict. Parents are naturally selected to maximize the difference between the benefits and the costs, and parental care will tend to exist when the benefits are substantially greater than the costs. 

Parents are equally related to all offspring, and so in order to optimize their fitness and chance of reproducing their genes, they should distribute their investment equally among current and future offspring. However, any single offspring is more related to themselves (they have 100% of their DNA in common with themselves) than they are to their siblings (siblings usually share 50% of their DNA), it is best for the offspring's fitness if the parent(s) invest more in them. To optimize fitness, a parent would want to invest in each offspring equally, but each offspring would want a larger share of parental investment. The parent is selected to invest in the offspring up until the point at which investing in the current offspring is costlier than investing in future offspring.

In iteroparous species, where individuals may go through several reproductive bouts during their lifetime, a tradeoff may exist between investment in current offspring and future reproduction. Parents need to balance their offspring's demands against their own self-maintenance. This potential negative effect of parental care was explicitly formalized by Trivers in 1972, who originally defined the term parental investment to mean any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.

King penguin and a chick

Penguins are a prime example of species that drastically sacrifices their own health and well-being in exchange for the survival of their offspring. This behavior, one that does not necessarily benefit the individual, but the genetic code from which the individual arises, can be seen in the King Penguin. Although some animals do exhibit altruistic behaviors towards individuals that are not of direct relation, many of these behaviors appear mostly in parent-offspring relationships. While breeding, males remain in a fasting-period at the breeding site for five weeks, waiting for the female to return for her own incubation shift. However, during this time period, males may decide to abandon their egg if the female is delayed in her return to the breeding grounds.

It shows that these penguins initially show a trade-off of their own health, in hopes of increasing the survivorship of their egg. But there comes a point where the male penguin's costs become too high in comparison to the gain of a successful breeding season. Olof Olsson investigated the correlation between how many experiences in breeding an individual has and the duration an individual will wait until abandoning his egg. He proposed that the more experienced the individual, the better that individual will be at replenishing his exhausted body reserves, allowing him to remain at the egg for a longer period of time.

The males' sacrifice of their body weight and possible survivorship, in order to increase their offspring's chance of survival is a trade-off between current reproductive success and the parents' future survival. This trade-off makes sense with other examples of kin-based altruism and is a clear example of the use of altruism in an attempt to increase overall fitness of an individual's genetic material at the expense of the individual's future survival.

Maternal-offspring conflict in investment

The maternal-offspring conflict has also been studied in animals species and humans. One such case has been documented in the mid-1970s by ethologist Wulf Schiefenhövel. Eipo women of West New Guinea engage in a cultural practice in which they give birth just outside the village. Following the birth of their child, each woman weighed whether or not she should keep the child or leave the child in the brush nearby, inevitably ending in the death of the child. Likelihood of survival and availability of resources within the village were factors that played into this decision of whether or not to keep the baby. During one illustrated birth, the mother felt the child was too ill and would not survive, so she wrapped the child up, preparing to leave the child in the brush; however, upon seeing the child moving, the mother unwrapped the child and brought it into the village, demonstrating a shift of life and death. This conflict between the mother and the child resulted in detachment behaviors in Brazil, seen in Scheper-Hughes work as "many Alto babies remain[ed] not only unchristened but unnamed until they begin to walk or talk", or if a medical crisis arose and the baby needed an emergency baptism. This conflict between survival, both emotional and physical, prompted a shift in cultural practices, thus resulting in new forms of investment from the mother towards the child.

Alloparental care

Alloparental care also referred to as 'Allomothering,' is when a member of a community, apart from the biological parents of the infant, partake in offspring care provision. A range of behaviors fall under the term alloparental care, some of which are: carrying, feeding, watching over, protecting, and grooming. Through alloparental care stress on parents, especially the mother, can be reduced, therefore reducing the negative effects of the parent-offspring conflict on the mother. In While the apparent altruistic nature of the behavior may seem at odds with Darwin's theory of natural selection, as taking care of offspring which are not one's own would not increase one's direct fitness, while taking time, energy and resources away from raising one's own offspring, the behavior can be explained evolutionarily as increasing indirect fitness, as the offspring is likely to be non-descendent kin, therefore carrying some of the genetics of the alloparent.

Offspring and situation direction

Parental investment behavior enhances the chances of survival of offspring, and it does not require underlying mechanisms to be compatible with empathy applicable to adults, or situations involving unrelated offspring, and it does not require the offspring to reciprocate the altruistic behavior in any way. Parentally investing individuals are not more vulnerable to being exploited by other adults.

Trivers' parental investment theory

Parental investment as defined by Trivers in 1972 is the investment in offspring by the parent that increases the offspring's chances of surviving and hence reproductive success at the expense of the parent's ability to invest in other offspring. A large parental investment largely decreases the parents' chances of investing in other offspring. Parental investment can be split into two main categories: mating investment and rearing investment. Mating investment consist of the sexual act and the sex cells invested. The rearing investment is the time and energy expended to raise the offspring after conception. Women's parental investment in both mating and rearing efforts greatly surpasses that of the male. In terms of sex cells (egg and sperms cells), the female's investment is a lot larger, while males produce thousands of sperm cells which are supplied at a rate of twelve million per hour.

Women have a fixed supply of around 400 ova. Also, the acts of fertilization and gestation occur in the women, which compared to the male's investment of just one cell outweighs it. Furthermore, each intercourse could result in a nine-month commitment such as gestation (the act of breastfeeding) for the woman. From Trivers' theory of parental investment several implications follow. The first is that women are often but not always the more investing sex. The fact that they are the more investing sex has meant that evolution has favored females who are more selective of their mates to ensure that intercourse would not result in unnecessary costs. The third implication is that because women invest more and are essential for the reproductive success of their offspring they are a valuable resource for men; as a result, males often compete for sexual access to them.

Males as the more investing sex

For many species the only type of male investment received is that of sex cells. In those terms, the female investment greatly exceeds that of male investment as previously mentioned. However, there are other ways in which males invest in their offspring. For example, the male can find food as in the example of balloon flies. He may find a safe environment for the female to feed or lay her eggs as exemplified in many birds.

He may also protect the young and provide them with opportunities to learn as in many young as in wolves. Overall, the main role that males overtake is that of protection of the female and their young. That often can decrease the discrepancy of investment caused by the initial investment of sex cells. There are some species such as the Mormon cricket, pipefish seahorse and Panamanian poison arrow frog males invest more. Among the species where the male invests more, the male is also the pickier sex, placing higher demands on their selected female. For example, the female that they often choose usually contain 60% more eggs than rejected females.

This links Parental Investment Theory (PIT) with sexual selection: where parental investment is bigger for a male than a female, it's usually the female who competes for a mate, as shown by Phalaropidae and polyandrous bird species. In these species females are usually more aggressive, brightly colored, and larger than males, suggesting the more investing sex has more choice while selecting a mate compared to the sex engaged in intra-sexual selection.

Females as a valuable resource for males

The second prediction that follows from Trivers' theory is that the fact that women invest more heavily in offspring makes them a valuable resource for males as it ensures the survival of their offspring which is the driving force of natural selection. Therefore, the sex that invests less in offspring will compete among themselves to breed with the more heavily investing sex. In other words, males will compete for females. It has been argued that jealousy has developed to avert the risk of potential loss of parental investment in offspring.

If a male redirects his resources to another female it is a costly loss of time, energy and resources for her offspring. However, the risks for males are higher because although women invest more in their offspring, they have bigger maternity certainty because they themselves have carried out the child. However, males can never have 100% paternal certainty and therefore risk investing resources and time in offspring that is genetically unrelated. Evolutionary psychology views jealousy as an adaptive response to this problem.

Application of Trivers' theory in real life

Trivers' theory has been very influential as the predictions it makes correspond to differences in sexual behaviors of men and women, as demonstrated by a variety of research. Cross-cultural study from Buss (1989) shows that males are tuned into physical attractiveness as it signals youth and fertility and ensures male reproductive success, which is increased by copulating with as many fertile females as possible. Women on the other hand are tuned into resources provided by potential mates, as their reproductive success is increased by ensuring their offspring will survive, and one way they do so is by getting resources for them. Alternatively, another study shows that men are more promiscuous than women, giving further support to this theory. Clark and Hatfield found that 75% of men were willing to have sex with a female stranger when propositioned, compared to 0% of women. On the other hand, 50% of women agreed to a date with a male stranger. This suggests males seek short term relationships, while women show a strong preference for long-term relationships.

However, these preferences (male promiscuity and female choosiness) can be explained in other ways. In Western cultures, male promiscuity is encouraged through the availability of pornographic magazines and videos targeted to the male audience. Alternatively, both Western and Eastern cultures discourage female promiscuity through social checks such as slut-shaming.

PIT (Parental Investment Theory) also explains patterns of sexual jealousy. Males are more likely to show a stress response when imagining their partners showing sexual infidelity (having sexual relations with someone else), and women showed more stress when imagining their partner being emotionally unfaithful (being in love with another woman). PIT explains this, as woman's sexual infidelity decreases the male's paternal certainty, thus he will show more stress due to fear of cuckoldry. On the other hand, the woman fears losing the resources her partner provides. If her partner has an emotional attachment to another female it's likely that he won't invest into their offspring as much, thus a greater stress response is shown in this circumstance.

A heavy criticism of the theory comes from Thornhill and Palmer's analysis of it in A Natural History of Rape: Biological Bases of Sexual Coercion, as it seems to rationalize rape and sexual coercion of females. Thornhill and Palmer claimed rape is an evolved technique for obtaining mates in an environment where women choose mates. As PIT claims males seek to copulate with as many fertile females as possible, the choice women have could result in a negative effect on the male's reproductive success. If women didn't choose their mates, Thornhill and Palmer claim there would be no rape. This ignores a variety of sociocultural factors, such as the fact that not only fertile females are raped – 34% of underage rape victims are under 12, which means they are not of fertile age, thus there is no evolutionary advantage in raping them. 14% of rapes in England are committed on males, who cannot increase a man's reproductive success as there will be no conception. Thus, what Thornhill and Palmer called an 'evolved machinery' might not be very advantageous.

Versus sexual strategies

Trivers' theory overlooks that women do have short-term relationships such as one-night stands, while not all men behave promiscuously. An alternative explanation to PIT (Parental Investment Theory) and mate preferences would be Buss and Schmitt's sexual strategies theory. SST argues that both sexes pursue short-term and long-term relationships, but seek different qualities in their short- and long-term partners. For a short-term relationship women will prefer an attractive partner, but in a long-term relationship they might be willing to trade-off that attractiveness for resources and commitment. On the other hand, men might be accepting of a sexually willing partner in a short-term relationships, but to ensure their paternal certainty they will seek a faithful partner instead.

International politics

Parental investment theory is not only used to explain evolutionary phenomena and human behavior but describes recurrences in international politics as well. Specifically, parental investment is referred to when describing competitive behaviors between states and determining aggressive nature of foreign policies. The parental investment hypothesis states that the size of coalitions and the physical strengths of its male members determines whether its activities with its foreign neighbors are aggressive or amiable. According to Trivers, men have had relatively low parental investments, and were therefore forced into fiercer competitive situations over limited reproductive resources. Sexual selection naturally took place and men have evolved to address its unique reproductive problems. Among other adaptations, men's psychology has also developed to directly aid men in such intra-sexual competition.

One essential psychological developments involved decision-making of whether to take flight or actively engage in warfare with another rivalry group. The two main factors that men referred to in such situations were (1) whether the coalition they are a part of is larger than its opposition and (2) whether the men in their coalition have greater physical strength than the other. The male psychology conveyed in the ancient past has been passed on to modern times causing men to partly think and behave as they have during ancestral wars. According to this theory, leaders of international politics were not an exception. For example, the United States expected to win the Vietnam war due to its greater military capacity when compared to its enemies. Yet victory, according to the traditional rule of greater coalition size, did not come about because the U.S. did not take enough consideration to other factors, such as the perseverance of the local population.

The parental investment hypothesis contends that male physical strength of a coalition still determines the aggressiveness of modern conflicts between states. While this idea may seem unreasonable upon considering that male physical strength is one of the least determining aspects of today's warfare, human psychology has nevertheless evolved to operate on this basis. Moreover, although it may seem that mate seeking motivation is no longer a determinant, in modern wars sexuality, such as rape, is undeniably evident in conflicts even to this day.

Pair of crested auklets

Sexual selection

In many species, males can produce a larger number of offspring over the course of their lives by minimizing parental investment in favor of investing time impregnating any reproductive-age female who is fertile. In contrast, a female can have a much smaller number of offspring during her reproductive life, partly due to higher obligate parental investment. Females will be more selective ("choosy") of mates than males will be, choosing males with good fitness (e.g., genes, high status, resources, etc.), so as to help offset any lack of direct parental investment from the male, and therefore increase reproductive success. Robert Trivers' theory of parental investment predicts that the sex making the largest investment in lactation, nurturing, and protecting offspring will be more discriminating in mating; and that the sex that invests less in offspring will compete via intrasexual selection for access to the higher-investing sex.

In species where both sexes invest highly in parental care, mutual choosiness is expected to arise. An example of this is seen in crested auklets, where parents share equal responsibility in incubating their single egg and raising the chick. In crested auklets both sexes are ornamented.

Parental investment in humans

Humans have evolved increasing levels of parental investment, both biologically and behaviorally. The fetus requires high investment from the mother, and the altricial newborn requires high investment from a community. Species whose newborn young are unable to move on their own and require parental care have a high degree of altriciality. Human children are born unable to care for themselves and require additional parental investment post-birth in order to survive.

Maternal investment

Trivers (1972) hypothesized that greater biologically obligated investment will predict greater voluntary investment. Mothers invest an impressive amount in their children before they are even born. The time and nutrients required to develop the fetus, and the risks associated with both giving these nutrients and undergoing childbirth, are a sizable investment. To ensure that this investment is not for nothing, mothers are likely to invest in their children after they are born, to be sure that they survive and are successful. Relative to most other species, human mothers give more resources to their offspring at a higher risk to their own health, even before the child is born. This is associated with the evolution of a slower life history, in which fewer, larger offspring are born after longer intervals, requiring increased parental investment.

The placenta attaches to the uterine wall, and the umbilical cord connects it to the fetus.

The developing human fetus––and especially the brain––requires nutrients to grow. In the later weeks of gestation, the fetus requires increasing nutrients as the growth of the brain increases. Rodents and primates have the most invasive placenta phenotype, the hemochorial placenta, in which the chorion erodes the uterine epithelium and has direct contact with maternal blood. The other placental phenotypes are separated from the maternal bloodstream by at least one layer of tissue. The more invasive placenta allows for a more efficient transfer of nutrients between the mother and fetus, but it comes with risks as well. The fetus is able to release hormones directly into the mother’s bloodstream to “demand” increased resources. This can result in health problems for the mother, such as pre-eclampsia. During childbirth, the detachment of the placental chorion can cause excessive bleeding.

The obstetrical dilemma also makes birth more difficult and results in increased maternal investment. Humans have evolved both bipedalism and large brain size. The evolution of bipedalism altered the shape of the pelvis, and shrunk the birth canal at the same time brains were evolving to be larger. The decreasing birth canal size meant that babies are born earlier in development, when they have smaller brains. Humans give birth to babies with brains 25% developed, while other primates give birth to offspring with brains 45-50% developed. A second possible explanation for the early birth in humans is the energy required to grow and sustain a larger brain. Supporting a larger brain gestationally requires energy the mother may be unable to invest.

The obstetrical dilemma makes birth challenging, and a distinguishing trait of humans is the need for assistance during childbirth. The altered shape of the bipedal pelvis requires that babies leave the birth canal facing away from the mother, contrary to all other primate species. This makes it more difficult for the mother to clear the baby’s breathing passageways, to make sure the umbilical cord isn’t wrapped around the neck, and to pull the baby free without bending its body the wrong way.

The human need to have a birth attendant also requires sociality. In order to guarantee the presence of a birth attendant, humans must aggregate in groups. It has been controversially claimed that humans have eusociality, like ants and bees, in which there is relatively high parental investment, cooperative care of young, and division of labor. It is unclear which evolved first; sociality, bipedalism, or birth attendance. Bonobos, our closest living relatives alongside chimpanzees, have high female sociality and births among bonobos are also social events. Sociality may have been a prerequisite for birth attendance, and bipedalism and birth attendance could have evolved as long as five million years ago.

A baby, mother, grandmother, and great-grandmother. In humans, grandparents often help to raise a child.

 

As female primates age, their ability to reproduce decreases. The grandmother hypothesis describes the evolution of menopause, which may or may not be unique to humans among primates. As women age, the costs of investing in additional reproduction increase and the benefits decrease. At menopause, it is more beneficial to stop reproduction and begin investing in grandchildren. Grandmothers are certain of their genetic relation to their grandchildren, especially the children of their daughters, because maternal certainty of their own children is high, and their daughters are certain of their maternity to their children as well. It has also been theorized that grandmothers preferentially invest in the daughters of their daughters because X chromosomes carry more DNA and their granddaughters are most closely related to them.

Paternal investment

As altriciality increased, investment from individuals other than the mother became more necessary. High sociality meant that female relatives were present to help the mother, but paternal investment increased as well. Paternal investment increases as it becomes more difficult to have additional children, and as the effects of investment on offspring fitness increase.

Men are more likely than women to give no parental investment to their children, and the children of low-investing fathers are more likely to give less parental investment to their own children. Father absence is a risk factor for both early sexual activity and teenage pregnancy. Father absence raises children's stress levels, which are linked to earlier onset of sexual activity and increased short-term mating orientation. Daughters of absent fathers are more likely to seek short-term partners, and one theory explains this as a preference for outside (non-partner) social support because of the perceived uncertain future and uncertain availability of committing partners in a high-stress environment.

Investment as predictor of mating strategies

Chance of fertilization by menstrual cycle day relative to ovulation, with data from two different studies.

Concealed ovulation

Women can only get pregnant while ovulating. Human ovulation is concealed, or not signaled externally. Concealed ovulation decreases paternity certainty because men are unsure when women ovulate. The evolution of concealed ovulation has been theorized to be a result of altriciality and increased need for paternal investment. There are two ways this could be true. First, if men are unsure of the time of ovulation, the best way to successfully reproduce would be to repeatedly mate with a woman throughout her cycle, which requires pair bonding, which in turn increases paternal investment. The second theory states that decreased paternity certainty would increase paternal investment in polygamous groups, because more men may invest in the offspring. The second theory is better regarded today, because all mammals with concealed ovulation are promiscuous, and men display relatively low mate-guarding behavior, as monogamy and the first theory require.

Mating orientations

Sociosexuality was first described by Alfred Kinsey as a willingness to engage in casual and uncommitted sexual relationships. Sociosexual orientation describes sociosexuality on a scale from unrestricted to restricted. Individuals with an unrestricted sociosexual orientation have higher openness to sex in less committed relationships, and individuals with a restricted sociosexual orientation have lower openness to casual sexual relationships. However, today it is acknowledged that sociosexuality does not in reality exist on a one-dimensional scale. Individuals who are less open to casual relationships are not always seeking committed relationships, and individuals who are less interested in committed relationships are not always interested in casual relationships. Short- and long-term mating orientations are the modern descriptors of openness to uncommitted and committed relationships, respectively.

Parental investment theory, as proposed by Trivers, argues that the sex with higher obligatory investment will be more selective in choosing sex partners, and the sex with lower obligatory investment will be less selective and more interested in "casual" mating opportunities. The more investing sex cannot reproduce as frequently, causing the less investing sex to compete for mating opportunities. In humans, women have higher obligatory investment (pregnancy and childbirth), than men (sperm production). Women are more likely to have higher long-term mating orientations, and men are more likely to have higher short-term mating orientations.

Short- and long-term mating orientations influence women's preferences in men. Studies have found that women put great emphasis on career-orientation, ambition and devotion only when considering a long-term partner. When marriage is not involved, women put greater emphasis on physical attractiveness. Generally, women prefer men who are likely to perform high parental investment and have good genes. Women prefer men with good financial status, who are more committed, who are more athletic, and who are healthier.

Some inaccurate theories have been inspired by parental investment theory. The "structural powerlessness hypothesis" proposes that women strive to find mates with access to high levels of resources because as women, they are excluded from these resources directly. However, this hypothesis has been disproved by studies which found that financially successful women place an even greater importance on financial status, social status, and possession of professional degrees.

Humans are sexually dimorphic; the average man is taller than the average woman.

Sexual dimorphism

Sexual dimorphism is the difference in body size between male and female members of a species as a result of intrasexual selection, which is sexual selection that acts within a sex. High sexual dimorphism and larger body size in males is a result of male-male competition for females. Primate species in which groups are formed of many females and one male have higher sexual dimorphism than species that have both multiple females and males, or one female and one male. Polygynous primates have the highest sexual dimorphism, and polygamous and monogamous primates have less. Humans have the lowest levels of sexual dimorphism of any primate species, indicating that we have evolved decreasing levels of polygyny. Decreased polygyny is associated with increased paternal investment.

The demographic transition

The demographic transition describes the modern decrease in both birth and death rates. From a Darwinian perspective, it does not make sense that families with more resources are having fewer children. One explanation for the demographic transition is the increased parental investment required to raise children who will be able to maintain the same level of resources as their parents.

Sexual selection in humans

From Wikipedia, the free encyclopedia

Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection, as it affects humans. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that the human brain is itself a product of sexual selection, i.e. it has developed as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model. Fisher also stated that the development of sexual selection was "more favourable" in humans.

General hypotheses

Some hypotheses about the evolution of the human brain argue that it is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs (a quarter to a fifth of the energy and oxygen consumed by a human).

Sexual selection's role in human evolution cannot be definitively established, as features may result from an equilibrium among competing selective pressures, some involving sexual selection, others natural selection, and others pleiotropy. In the words of Richard Dawkins:

When you notice a characteristic of an animal and ask what its Darwinian survival value is, you may be asking the wrong question. It could be that the characteristic you have picked out is not the one that matters. It may have "come along for the ride", dragged along in evolution by some other characteristic to which it is pleiotropically linked.

Darwin's sexual selection hypothesis

Charles Darwin described sexual selection as depending on "the advantage which certain individuals have over others of the same sex and species, solely in respect of reproduction". Darwin noted that sexual selection is of two kinds and concluded that both kinds had operated on humans: "The sexual struggle is of two kinds; in the one it is between the individuals of the same sex, generally the male sex, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners."

Charles Darwin conjectured that the male beard, as well as the hairlessness of humans compared to nearly all other mammals, were results of sexual selection. He reasoned that since the bodies of females are more nearly hairless, the loss of fur was due to sexual selection of females at a remote prehistoric time when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes. He also hypothesized that contrasts in sexual selection acting along with natural selection were significant factors in the geographical differentiation in human appearance of some isolated groups, as he did not believe that natural selection alone provided a satisfactory answer. Although not explicit, his observation that in Khoisan women "the posterior part of the body projects in a most wonderful manner" (known as steatopygia) implies sexual selection for this characteristic. In The Descent of Man, and Selection in Relation to Sex, Darwin viewed many physical traits which vary around the world as being so trivial to survival that he concluded some input from sexual selection was required to account for their presence. He noted that variation in these features among the various peoples of the world meant human mate-choice criteria would also have to be quite different if the focus was similar, and he himself doubted that, citing reports indicating that ideals of beauty did not, in fact, vary in this way around the world.

Sexual dimorphism

Men are generally hairier than women, and Darwin was of the opinion that hairlessness was related to sexual selection; however, several other explanations have been advanced to explain human hairlessness, a leading one is loss of body hair to facilitate sweating. This idea closely relates to that of the suggested need for increased photoprotection and is part of the most-commonly-accepted scientific explanation for the evolution of pigmentary traits.

Indicating that a trait is under sexual selection can be difficult to prove through correlational methods, as characters may result from different selective pressures, some involving sexual selection, others natural selection, and some may be accidental and due to pleiotropy. For example, monogamous primates are known to typically exhibit little sexual dimorphism such as particularly large males armed with huge canines; however, powerful big-toothed males can provide protection against predators and may be bigger for that reason, rather than in order to win confrontations over females. Males and females differing in size can specialize in, and more fully exploit, different food resources while avoiding competing with each other; furthermore, body size can be useful in avoiding predators and may also be of assistance in securing a mate. This is further complicated by the consideration that with larger body size, the skeleton of mammals becomes much more robust and massive (relatively speaking). Bearing these caveats in mind, levels of sexual dimorphism are generally seen as a marker of sexual selection. Studies have shown the earliest homininae were highly dimorphic and that this tendency lessened over the course of human evolution, suggesting humans have become more monogamous. In contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae).

Sexual anatomy

The theory of sexual selection has been used to explain a number of human anatomical features. These include rounded breasts, facial hair, pubic hair and penis size. The breasts of primates are flat, yet are able to produce sufficient milk for feeding their young. The breasts of non-lactating human females are filled with fatty tissue and not milk. Thus it has been suggested the rounded female breasts are signals of fertility. Richard Dawkins has speculated that the loss of the penis bone in humans, when it is present in other primates, may be due to sexual selection by females looking for a clear sign of good health in prospective mates. Since a human erection relies on a hydraulic pumping system, erection failure is a sensitive early warning of certain kinds of physical and mental ill health.

Homo has a thicker penis than the other great apes, though it is no longer than the chimpanzee's. It has been suggested the evolution of the human penis towards larger size was the result of female choice rather than sperm competition, which generally favors large testicles. However, penis size may have been subject to natural selection, rather than sexual selection, due to a larger penis' efficiency in displacing the sperm of rival males during sexual intercourse. A model study showed displacement of semen was directly proportional to the depth of pelvic thrusting, as an efficient semen displacement device.

Selection preferences and biological drivers

There are a variety of factors that drive sexual selection in humans. Current available research indicates that selection preferences are biologically driven, that is, by the display of phenotypic traits that can be both consciously and unconsciously evaluated by the opposite sex to determine the health and fertility of a potential mate. This process can be affected, however, by social factors, including in cultures where arranged marriage is practiced, or psychosocial factors, such as valuing certain cultural traits of a mate, including a persons social status, or what is perceived to be an ideal partner in various cultures.

Selection preferences in females

Some of the factors that affect how females select their potential mates for reproduction include voice pitch, facial shape, muscular appearance, and height. Several studies suggest that there is a link between hormone levels and partner selection among humans. In a study measuring female attraction to males with varying levels of masculinity, it was established that women had a general masculinity preferences for men's voices, and that the preference for masculinity was greater in the fertile phase of the menstrual cycle than in the non-fertile phase. There is further evidence from the same study that in fertile stages of the menstrual cycle, women also had a preference for other masculine traits such as body size, facial shape, and dominant behavior, which are indicators of both fertility and health. This study did not exclude males with feminine traits from being selected, however, as feminine traits in men indicate a higher probability of long-term relationship commitment, and may be one of several survival strategies. Further research also backs up the idea of using phenotypic traits as a means of assessing a potential mates fitness for reproduction as well as assessing whether a partner has high genetic quality.

Another factor affecting the selection process is the environment which the person inhabits. In biological terms, certain environmental conditions may bring about demands for or the disregarding of certain traits. One such example is a preference for males whose facial structure indicates certain hormonal ratios, such as testosterone-cortisol levels (sex and stress hormones). Research shows that, for example, in countries with varying Human Development Index (HDI) levels, females have different preferences for sex-stress hormone ratios, as expressed in the male's face. A Royal Society research showed a significant correlation between a measure of societal development and preferences for indication of higher testosterone levels, as manifested in facial features, and the interaction between preferences for testosterone and cortisol." It was concluded that societal-level ecological factors impact the valuation of traits by combinations of sex- and stress-hormones.

Selection preferences in males

Like their female counterparts, males also use visual information about a potential mate, as well as voice, body shape, and an assortment of other factors in selecting a partner. Research shows that males tend to prefer feminine women's faces and voices as opposed to women with masculine features in these categories. Furthermore, males also evaluate skin coloration, symmetry, and apparent health, as a means by which the select a partner for reproductive purposes. Males are particularly attracted to femininity in women's faces when their testosterone levels are at their highest, and the level of attraction to femininity may fluctuate as hormone levels fluctuate. Studies on men have also been done to show the effects of exogenous testosterone and its effects on attraction to femininity, and the results concluded that throughout several studies, men have shown decreased preference for feminine female faces in the long-term context, when given exogenous testosterone, but this difference did not occur with placebo.

Common preferences in either sex

Sexual selection preferences are general terms by which the mating and reproductive process are understood. As one article states, sexual selection is in essence a process which favors sexual displays for attraction, aggressiveness, dominance, size, and strength, and the ability to exclude competitors by force if necessary, or by using resources to win. Both male and female use voice, face, and other physical characteristics to assess a potential mate's ability to reproduce, as well as their health. Together with visual and chemical signals, these crucial characteristics which are likely to enhance the ability to produce offspring, as well as long term survival prospects, can be assessed and selections made.

Phenotype

Sexual selection has continued to be suggested as a possible explanation for geographical variation in appearance within the human species; in modern hypotheses, marriage practices are proposed as the main determinant of sexual selection. John Manning suggests that where polygyny is common, men face intense competition for wives and are more likely to be completely unsuccessful in reproducing, and the result is strong selection of males for traits which are adaptive for successful reproduction. He proposes a link to skin color through selection of males for testosterone-mediated traits which confer an ability to successfully compete for females. He suggests testosterone makes the human immune system less competent to resist pathogens. In this view the antimicrobial properties of melanin help mitigate the susceptibility to disease that polygyny induces by increasing testosterone. According to this argument, the anti-infective qualities of melanin were more important than protection from ultraviolet light in the evolution of the darkest skin types. Manning asserts that skin color is more correlated with the occurrence of polygyny – explicable by it having an antimicrobial function – than the latitudinal gradient in intensity of ultraviolet radiation, and he points to the lack of very dark skin at equatorial latitudes of the New World and the relatively light skin of Khoisan people in Africa.

Research seems to contradict Manning's explanation about skin color. In 1978, NASA launched the Total Ozone Mapping Spectrometer, which was able to measure the ultraviolet radiation reaching Earth's surface. Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. There was an unmistakable correlation: The weaker the ultraviolet light, the fairer the skin. Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences of chimpanzees and humans from various regions of the Earth. Rogers concluded that, at the time of the evolutionary separation of chimpanzees and humans, the common ancestors of all humans had light skin that was covered by dark hair. Additionally, our closest extant relative, the chimpanzee, has light skin covered by thick body hair. Over time human hair disappeared to allow better heat dissipation through sweating and the skin tone grew darker to increase the epidermal permeability barrier and protect from folate depletion due to the increased exposure to sunlight. When humans started to migrate away from the tropics, there was less-intense sunlight, partly due to clothing to protect against cold weather. Under these conditions there was less photo-destruction of folate, and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. The genetic mutations leading to light skin experienced selective pressure due to settlement in northern latitudes.

Anthropologist Peter Frost has proposed that sexual selection was responsible for the evolution of pigmentary traits of women in Northern and Eastern European populations. He contends that the diversity of hair and eye color in Northeast European populations originated as a consequence of intense female-female competition, and is an adaptation for reproductive success in women.

Geoffrey Miller hypothesis

Homo habilis - Forensic facial reconstruction

Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behavior, has hypothesized that many human behaviors not clearly tied to survival benefits, such as humor, music, visual art, some forms of altruism, verbal creativity or the fact that most humans have a far greater vocabulary than that which is required for survival, Miller (2000) has proposed that this apparent redundancy is due to individuals using vocabulary to demonstrate their intelligence, and consequently their "fitness", to potential mates. This has been tested experimentally, and it appears that males do make greater use of lower-frequency (more unusual) words when in a romantic mindset compared to a non-romantic mindset, suggesting that vocabulary is likely to be used as a sexual display (Rosenberg & Tunney, 2008). All these qualities are considered courtship adaptations that have been favored through sexual selection.

Miller is critical of theories that imply that human culture arose as accidents or by-products of human evolution. He believes that human culture arose through sexual selection for creative traits. In that view, many human artifacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhances sexually selected traits. During human evolution, on at least two occasions, hominid brain size increased rapidly over a short period of time followed by a period of stasis. The first period of brain expansion occurred 2.5 million years ago, when Homo habilis first began using stone tools. The second period occurred 500,000 years ago, with the emergence of archaic Homo sapiens. Miller argues that the rapid increases in brain size would have occurred by a positive feedback loop resulting in a Fisherian runaway selection for larger brains. Tor Nørretranders, in The Generous Man conjectures how intelligence, musicality, artistic and social skills, and language might have evolved as an example of the handicap principle, analogously with the peacock's tail, the standard example of that principle. Another hypothesis proposes that human intelligence is a courtship indicator of health and resistance against parasites and pathogens which are deleterious to human cognitive capabilities.

Opposing arguments

The role of sexual selection in human evolution has been considered controversial from the moment of publication of Darwin's book on sexual selection (1871). Among his vocal critics were some of Darwin's supporters, such as Alfred Wallace, who argued that animals and birds do not choose mates based on their beauty or fine plumages, and that the artistic faculties in humans belong to their spiritual nature and therefore cannot be connected to natural selection, which only affects the animal nature. Darwin was accused of looking to the evolution of early human ancestors through the moral codes of the 19th century Victorian society. Joan Roughgarden, citing elements of sexual behavior in animals and humans that cannot be explained by the sexual-selection model, suggested that the function of sex in human evolution was primarily social.

Joseph Jordania suggested in 2011 that in explaining such human morphological and behavioral characteristics as singing, dancing, body painting, wearing of clothes, Darwin and proponents of sexual selection neglect another important evolutionary force, intimidation of predators and competitors with the ritualized forms of warning display, which uses the same arsenal of visual, audio, olfactory and behavioral features as sexual selection. According to Jordania, most of these warning displays were incorrectly attributed to the forces of sexual selection. Jordania proposed an aposematic model of human evolution, where most of the human morphological and behavioral features that had been considered by Darwin as the result of sexual selection, via female choice, are explained by the aposematic (intimidating) display.