Mind–body problem

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https://en.wikipedia.org/wiki/Mind%E2%80%93body_problem 

Illustration of mind–body dualism by René Descartes. Inputs are passed by the sensory organs to the pineal gland, and from there to the immaterial spirit.

The mind–body problem is a philosophical problem concerning the relationship between thought and consciousness in the human mind and body. It addresses the nature of consciousness, mental states, and their relation to the physical brain and nervous system. The problem centers on understanding how immaterial thoughts and feelings can interact with the material world, or whether they are ultimately physical phenomena.

This problem has been a central issue in philosophy of mind since the 17th century, particularly following René Descartes' formulation of dualism, which proposes that mind and body are fundamentally distinct substances. Other major philosophical positions include monism, which encompasses physicalism (everything is ultimately physical) and idealism (everything is ultimately mental). More recent approaches include functionalism, property dualism, and various non-reductive theories.

The mind-body problem raises fundamental questions about causation between mental and physical events, the nature of consciousness, personal identity, and free will. It remains significant in both philosophy and science, influencing fields such as cognitive science, neuroscience, psychology, and artificial intelligence.

In general, the existence of these mind–body connections seems unproblematic. Issues arise, however, when attempting to interpret these relations from a metaphysical or scientific perspective. Such reflections raise a number of questions, including:

• Are the mind and body two distinct entities, or a single entity?
• If the mind and body are two distinct entities, do the two of them causally interact?
• Is it possible for these two distinct entities to causally interact?
• What is the nature of this interaction?
• Can this interaction ever be an object of empirical study?
• If the mind and body are a single entity, then are mental events explicable in terms of physical events, or vice versa?
• Is the relation between mental and physical events something that arises de novo at a certain point in development?

These and other questions that discuss the relation between mind and body are questions that all fall under the banner of the 'mind–body problem'.

Mind–body interaction and mental causation

Philosophers David L. Robb and John F. Heil introduce mental causation in terms of the mind–body problem of interaction:

Mind–body interaction has a central place in our pretheoretic conception of agency. Indeed, mental causation often figures explicitly in formulations of the mind–body problem. Some philosophers insist that the very notion of psychological explanation turns on the intelligibility of mental causation. If your mind and its states, such as your beliefs and desires, were causally isolated from your bodily behavior, then what goes on in your mind could not explain what you do. If psychological explanation goes, so do the closely related notions of agency and moral responsibility. Clearly, a good deal rides on a satisfactory solution to the problem of mental causation [and] there is more than one way in which puzzles about the mind's "causal relevance" to behavior (and to the physical world more generally) can arise.

[René Descartes] set the agenda for subsequent discussions of the mind–body relation. According to Descartes, minds and bodies are distinct kinds of "substance". Bodies, he held, are spatially extended substances, incapable of feeling or thought; minds, in contrast, are unextended, thinking, feeling substances. If minds and bodies are radically different kinds of substance, however, it is not easy to see how they "could" causally interact. Princess Elizabeth of Bohemia puts it forcefully to him in a 1643 letter:

how the human soul can determine the movement of the animal spirits in the body so as to perform voluntary acts—being as it is merely a conscious substance. For the determination of movement seems always to come about from the moving body's being propelled—to depend on the kind of impulse it gets from what sets it in motion, or again, on the nature and shape of this latter thing's surface. Now the first two conditions involve contact, and the third involves that the impelling thing has extension; but you utterly exclude extension from your notion of soul, and contact seems to me incompatible with a thing's being immaterial...

Elizabeth is expressing the prevailing mechanistic view as to how causation of bodies works. Causal relations countenanced by contemporary physics can take several forms, not all of which are of the push–pull variety.

— David Robb and John Heil, "Mental Causation" in The Stanford Encyclopedia of Philosophy

Contemporary neurophilosopher Georg Northoff suggests that mental causation is compatible with classical formal and final causality.

Biologist, theoretical neuroscientist and philosopher, Walter J. Freeman, suggests that explaining mind–body interaction in terms of "circular causation" is more relevant than linear causation.

In neuroscience, much has been learned about correlations between brain activity and subjective, conscious experiences. Many suggest that neuroscience will ultimately explain consciousness: "...consciousness is a biological process that will eventually be explained in terms of molecular signaling pathways used by interacting populations of nerve cells..." However, this view has been criticized because consciousness has yet to be shown to be a process, and the "hard problem" of relating consciousness directly to brain activity remains elusive.

Cognitive science today gets increasingly interested in the embodiment of human perception, thinking, and action. Abstract information processing models are no longer accepted as satisfactory accounts of the human mind. Interest has shifted to interactions between the material human body and its surroundings and to the way in which such interactions shape the mind. Proponents of this approach have expressed the hope that it will ultimately dissolve the Cartesian divide between the immaterial mind and the material existence of human beings (Damasio, 1994; Gallagher, 2005). A topic that seems particularly promising for providing a bridge across the mind–body cleavage is the study of bodily actions, which are neither reflexive reactions to external stimuli nor indications of mental states, which have only arbitrary relationships to the motor features of the action (e.g., pressing a button for making a choice response). The shape, timing, and effects of such actions are inseparable from their meaning. One might say that they are loaded with mental content, which cannot be appreciated other than by studying their material features. Imitation, communicative gesturing, and tool use are examples of these kinds of actions.

— Georg Goldenberg, "How the Mind Moves the Body: Lessons From Apraxia" in Oxford Handbook of Human Action

Since 1927, at the Solvay Conference in Austria, European physicists of the late 19th and early 20th centuries realized that the interpretations of their experiments with light and electricity required a different theory to explain why light behaves both as a wave and particle. The implications were profound. The usual empirical model of explaining natural phenomena could not account for this duality of matter and non-matter. In a significant way, this has brought back the conversation on the mind–body duality.

Another way of investigating the mind–body problem is to distinguish between first-person subjective experience (understood as a dynamic flow of constantly changing sensory experiences and mental events spanning the present moment, the past, and the future) and third-person accounts of phenomena in the world, including the body, the brain, and other observable entities. This understanding links to the realization that understanding the mind and understanding material phenomena (including the body and the brain), represent contrasting but equally valid epistemological frameworks.

Neural correlates

Main article: Neural correlates of consciousness

The neuronal correlates of consciousness constitute the smallest set of neural events and structures sufficient for a given conscious percept or explicit memory. This case involves synchronized action potentials in neocortical pyramidal neurons.

The neural correlates of consciousness "are the smallest set of brain mechanisms and events sufficient for some specific conscious feeling, as elemental as the color red or as complex as the sensual, mysterious, and primeval sensation evoked when looking at [a] jungle scene..." Neuroscientists use empirical approaches to discover neural correlates of subjective phenomena.

Neurobiology and neurophilosophy

Main articles: Neurobiology and Neurophilosophy

A science of consciousness must explain the exact relationship between subjective conscious mental states and brain states formed by electrochemical interactions in the body, the so-called hard problem of consciousness. Neurobiology studies the connection scientifically, as do neuropsychology and neuropsychiatry. Neurophilosophy is the interdisciplinary study of neuroscience and philosophy of mind. In this pursuit, neurophilosophers, such as Patricia Churchland, Paul Churchland and Daniel Dennett, have focused primarily on the body rather than the mind. In this context, neuronal correlates may be viewed as causing consciousness, where consciousness can be thought of as an undefined property that depends upon this complex, adaptive, and highly interconnected biological system. However, it's unknown if discovering and characterizing neural correlates may eventually provide a theory of consciousness that can explain the first-person experience of these "systems", and determine whether other systems of equal complexity lack such features.

The massive parallelism of neural networks allows redundant populations of neurons to mediate the same or similar percepts. Nonetheless, it is assumed that every subjective state will have associated neural correlates, which can be manipulated to artificially inhibit or induce the subject's experience of that conscious state. The growing ability of neuroscientists to manipulate neurons using methods from molecular biology in combination with optical tools was achieved by the development of behavioral and organic models that are amenable to large-scale genomic analysis and manipulation. Non-human analysis such as this, in combination with imaging of the human brain, have contributed to a robust and increasingly predictive theoretical framework.

Arousal and content

Midline structures in the brainstem and thalamus necessary to regulate the level of brain arousal. Small, bilateral lesions in many of these nuclei cause a global loss of consciousness.

There are two common but distinct dimensions of the term consciousness, one involving arousal and states of consciousness and the other involving content of consciousness and conscious states. To be conscious of something, the brain must be in a relatively high state of arousal (sometimes called vigilance), whether awake or in REM sleep. Brain arousal level fluctuates in a circadian rhythm but these natural cycles may be influenced by lack of sleep, alcohol and other drugs, physical exertion, etc. Arousal can be measured behaviorally by the signal amplitude required to trigger a given reaction (for example, the sound level that causes a subject to turn and look toward the source). High arousal states involve conscious states that feature specific perceptual content, planning and recollection or even fantasy. Clinicians use scoring systems such as the Glasgow Coma Scale to assess the level of arousal in patients with impaired states of consciousness such as the comatose state, the persistent vegetative state, and the minimally conscious state. Here, "state" refers to different amounts of externalized, physical consciousness: ranging from a total absence in coma, persistent vegetative state and general anesthesia, to a fluctuating, minimally conscious state, such as sleep walking and epileptic seizure.

Many nuclei with distinct chemical signatures in the thalamus, midbrain and pons must function for a subject to be in a sufficient state of brain arousal to experience anything at all. These nuclei therefore belong to the enabling factors for consciousness. Conversely it is likely that the specific content of any particular conscious sensation is mediated by particular neurons in the cortex and their associated satellite structures, including the amygdala, thalamus, claustrum and the basal ganglia.

Theoretical frameworks

Different approaches toward resolving the mind–body problem

A variety of approaches have been proposed. Most are either dualist or monist. Dualism maintains a rigid distinction between the realms of mind and matter. Monism maintains that there is only one unifying reality as in neutral or substance or essence, in terms of which everything can be explained.

Each of these categories contains numerous variants. The two main forms of dualism are substance dualism, which holds that the mind is formed of a distinct type of substance not governed by the laws of physics, and property dualism, which holds that mental properties involving conscious experience are fundamental properties, alongside the fundamental properties identified by a completed physics. The three main forms of monism are physicalism, which holds that the mind consists of matter organized in a particular way; idealism, which holds that only thought truly exists and matter is merely a representation of mental processes; and neutral monism, which holds that both mind and matter are aspects of a distinct essence that is itself identical to neither of them. Psychophysical parallelism is a third possible alternative regarding the relation between mind and body, between interaction (dualism) and one-sided action (monism).

Several philosophical perspectives that have sought to escape the problem by rejecting the mind–body dichotomy have been developed. The historical materialism of Karl Marx and subsequent writers, itself a form of physicalism, held that consciousness was engendered by the material contingencies of one's environment. An explicit rejection of the dichotomy is found in French structuralism, and is a position that generally characterized post-war Continental philosophy.

An ancient model of the mind known as the Five-Aggregate Model, described in the Buddhist teachings, explains the mind as continuously changing sense impressions and mental phenomena. Considering this model, it is possible to understand that it is the constantly changing sense impressions and mental phenomena (i.e., the mind) that experience/analyze all external phenomena in the world as well as all internal phenomena including the body anatomy, the nervous system as well as the organ brain. This conceptualization leads to two levels of analyses: (i) analyses conducted from a third-person perspective on how the brain works, and (ii) analyzing the moment-to-moment manifestation of an individual's mind-stream (analyses conducted from a first-person perspective). Considering the latter, the manifestation of the mind-stream is described as happening in every person all the time, even in a scientist who analyzes various phenomena in the world, including analyzing and hypothesizing about the organ brain.

Christian List argues that Benj Hellie's vertiginous question, i.e. why an individual exists as themselves and not as someone else, and the existence of first-personal facts, is evidence against physicalism. However, according to List, this is also evidence against other third-personal metaphysical pictures, including standard versions of dualism. List also argues that the vertiginous question implies a "quadrilemma" for theories of consciousness. He claims that at most three of the following metaphysical claims can be true: 'first-person realism', 'non-solipsism', 'non-fragmentation', and 'one world' – and thus one of these four must be rejected. List has proposed a model he calls the "many-worlds theory of consciousness" in order to reconcile the subjective nature of consciousness without lapsing into solipsism.

Dualism

The following is a very brief account of some contributions to the mind–body problem.

Interactionism

Main article: Interactionism (philosophy of mind)

The viewpoint of interactionism suggests that the mind and body are two separate substances, but that each can affect the other. This interaction between the mind and body was first put forward by the philosopher René Descartes. Descartes believed that the mind was non-physical and permeated the entire body, but that the mind and body interacted via the pineal gland. This theory has changed throughout the years, and in the 20th century its main adherents were the philosopher of science Karl Popper and the neurophysiologist John Carew Eccles. A more recent and popular version of Interactionism is the viewpoint of emergentism. This perspective states that mental states are a result of the brain states, and that the mental events can then influence the brain, resulting in a two way communication between the mind and body.

The absence of an empirically identifiable meeting point between the non-physical mind (if there is such a thing) and its physical extension (if there is such a thing) has been raised as a criticism of interactionalist dualism. This criticism has led many modern philosophers of mind to maintain that the mind is not something separate from the body. These approaches have been particularly influential in the sciences, particularly in the fields of sociobiology, computer science, evolutionary psychology, and the neurosciences.

Avshalom Elitzur has defended interactionism and has described himself as a "reluctant dualist". One argument Elitzur makes in favor of dualism is an argument from bafflement. According to Elitzur, a conscious being can conceive of a P-zombie version of his/herself. However, a P-zombie cannot conceive of a version of itself that lacks corresponding qualia.

Epiphenomenalism

Main article: Epiphenomenalism

A possible consequence of certain physicalist theories is epiphenomenalism. According to epiphenomenalism, mental events are causally dependent upon physical events, typically neural processes in the brain, but mental events themselves possess no causal efficacy with regard to physical processes. Thus, the experience of pain may be caused by neural activity, yet the subjective feeling of pain, strictly speaking, does not itself influence subsequent bodily events.

Non-reductionist physicalists attempt to avoid this seemingly counterintuitive conclusion by appealing to emergence. According to this perspective, once sufficiently complex brain activity gives rise to mental phenomena, these phenomena may exhibit so-called emergent properties, that is, properties that cannot be straightforwardly reduced to descriptions of neural firing patterns or other lower-level physical processes, even though they remain entirely dependent upon them.

This position, which may be considered the current standard (i.e. the overall most prevalent) position amongst most scientists and naturalistic philosophers, combines substance monism with a form of property dualism. It leaves conceptual room for phenomena such as free will, intentionality, and qualia, while at the same time remaining broadly committed to a naturalistic or materialist worldview. Critics, however, have argued that sufficiently strong versions of emergentism ultimately tend toward some variant of dualism, as genuinely novel causal powers appear difficult to reconcile with an exclusively physical ontology.

This viewpoint explains that while one's body may react to them feeling joy, fear, or sadness, that the emotion does not cause the physical response. Rather, it explains that joy, fear, sadness, and all bodily reactions are caused by chemicals and their interaction with the body.

Mind-brain identity theory

The Mind-Brain-Identity theory is a reductionist physicalist position, according to which mental states and brain states are not merely correlated but literally identical. There is no explanatory gap to bridge and no additional mystery to solve. The experience of pain, for example, is not simply associated with a particular neural process; it is that neural process, viewed from a different descriptive perspective.

Identity theory generally leaves little room for irreducible non-physical properties such as qualia and is typically compatible only with relatively weak notions of emergence, in the proverbial sense that the whole may exhibit patterns not immediately obvious from its constituent parts.

Nevertheless, since it appears difficult to deny the existence of subjective conscious experience altogether, some identity theorists have proposed that introspective awareness may be understood as a kind of neural self-monitoring, or, metaphorically speaking, as a form of proprioception of the brain by the brain itself. Precisely how such higher-order awareness emerges remains a topic of ongoing debate.

Psychophysical parallelism

Main article: Psychophysical parallelism

The viewpoint of psychophysical parallelism suggests that the mind and body are entirely independent from one another. Furthermore, this viewpoint states that both mental and physical stimuli and reactions are experienced simultaneously by both the mind and body, however, there is no interaction nor communication between the two.

Double aspectism

Main article: Double aspectism

Double aspectism is an extension of psychophysical parallelism which also suggests that the mind and body cannot interact, nor can they be separated. Baruch Spinoza and Gustav Fechner were two of the notable users of double aspectism, however, Fechner later expanded upon it to form the branch of psychophysics in an attempt to prove the relationship of the mind and body.

Pre-established harmony

The viewpoint of pre-established harmony is another offshoot of psychophysical parallelism which suggests that mental events and bodily events are separate and distinct, but that they are both coordinated by an external agent: an example of such an agent could be God. A notable adherent to the idea of pre-established harmony is Gottfried Wilhelm von Leibniz in his theory of Monadology. His explanation of pre-established harmony relied heavily upon God as the external agent who coordinated the mental and bodily events of all things in the beginning.

Gottfried Wilhelm Leibniz's theory of pre-established harmony (French: harmonie préétablie) is a philosophical theory about causation under which every "substance" affects only itself, but all the substances (both bodies and minds) in the world nevertheless seem to causally interact with each other because they have been programmed by God in advance to "harmonize" with each other. Leibniz's term for these substances was "monads", which he described in a popular work (Monadology §7) as "windowless".

The concept of pre-established harmony can be understood by considering an event with both seemingly mental and physical aspects. For example, consider saying 'ouch' after stubbing one's toe. There are two general ways to describe this event: in terms of mental events (where the conscious sensation of pain caused one to say 'ouch') and in terms of physical events (where neural firings in one's toe, carried to the brain, are what caused one to say 'ouch'). The main task of the mind–body problem is figuring out how these mental events (the feeling of pain) and physical events (the nerve firings) relate. Leibniz's pre-established harmony attempts to answer this puzzle, by saying that mental and physical events are not genuinely related in any causal sense, but only seem to interact due to psycho-physical fine-tuning.

Leibniz's theory is best known as a solution to the mind–body problem of how mind can interact with the body. Leibniz rejected the idea of physical bodies affecting each other, and explained all physical causation in this way.

Under pre-established harmony, the preprogramming of each mind must be extremely complex, since only it causes its own thoughts or actions, for as long as it exists. To appear to interact, each substance's "program" must contain a description of either the entire universe, or of how the object behaves at all times during all interactions that appear to occur.

An example:

An apple falls on Alice's head, apparently causing the experience of pain in her mind. In fact, the apple does not cause the pain—the pain is caused by some previous state of Alice's mind. If Alice then seems to shake her hand in anger, it is not actually her mind that causes this, but some previous state of her hand.

Note that if a mind behaves as a windowless monad, there is no need for any other object to exist to create that mind's sense perceptions, leading to a solipsistic universe that consists only of that mind. Leibniz seems to admit this in his Discourse on Metaphysics, section 14. However, he claims that his principle of harmony, according to which God creates the best and most harmonious world possible, dictates that the perceptions (internal states) of each monad "expresses" the world in its entirety, and the world expressed by the monad actually exists. Although Leibniz says that each monad is "windowless", he also claims that it functions as a "mirror" of the entire created universe.

On occasion, Leibniz styled himself as "the author of the system of pre-established harmony".

Immanuel Kant's professor Martin Knutzen regarded pre-established harmony as "the pillow for the lazy mind".

In his sixth Metaphysical Meditation, Descartes talked about a "coordinated disposition of created things set up by God", shortly after having identified "nature in its general aspect" with God himself. His conception of the relationship between God and his normative nature actualized in the existing world recalls both the pre-established harmony of Leibniz and the Deus sive Natura of Baruch Spinoza.

Occasionalism

Main article: Occasionalism

The viewpoint of Occasionalism is another offshoot of psychophysical parallelism, however, the major difference is that the mind and body have some indirect interaction. Occasionalism suggests that the mind and body are separate and distinct, but that they interact through divine intervention. Nicolas Malebranche was one of the main contributors to this idea, using it as a way to address his disagreements with Descartes' view of the mind–body problem. In Malebranche's occasionalism, he viewed thoughts as a wish for the body to move, which was then fulfilled by God causing the body to act.

A distinction between first- and third-person perspectives

Conceptually separating the first-person perspective from third-person understandings of phenomena in the world provides insight into distinguishing the mind from material phenomena (including the body, the brain, and other observable entities). From the first-person perspective, the mind can be conceptualized as a distinct and dynamic phenomenon consisting of constantly changing sensory encounters and mental processes involving the present moment, the past, and the future. In contrast, third-person scientific analysis relies on communication between individuals and the establishment of intersubjective agreement through shared observation and interpretation of material phenomena. While these two approaches to understanding reflect contrasting epistemological frameworks, insights derived from the first-person perspective foster self-knowledge and wisdom, whereas those arising from the third-person perspective illuminate the workings of the world.

Historical background

The problem was popularized by René Descartes in the 17th century, which resulted in Cartesian dualism, also by pre-Aristotelian philosophers, in Avicennian philosophy, and in earlier Asian traditions.

The Buddha

See also: Gautama Buddha, Buddhism and the body, and Pratītyasamutpāda

The Buddha (480–400 B.C.E), founder of Buddhism, described the mind and the body as depending on each other in a way that two sheaves of reeds were to stand leaning against one another and taught that the world consists of mind and matter which work together, interdependently. Buddhist teachings describe the mind as manifesting from moment to moment, one thought moment at a time as a fast flowing stream. The components that make up the mind are known as the five aggregates (i.e., material form, feelings, perception, volition, and sensory consciousness), which arise and pass away continuously. The arising and passing of these aggregates in the present moment is described as being influenced by five causal laws: biological laws, psychological laws, physical laws, volitional laws, and universal laws. The Buddhist practice of mindfulness involves attending to this constantly changing mind-stream.

Ultimately, the Buddha's philosophy is that both mind and forms are conditionally arising qualities of an ever-changing universe in which, when nirvāna is attained, all phenomenal experience ceases to exist. According to the anattā doctrine of the Buddha, the conceptual self is a mere mental construct of an individual entity and is basically an impermanent illusion, sustained by form, sensation, perception, thought and consciousness. The Buddha argued that mentally clinging to any views will result in delusion and stress, since, according to the Buddha, a real self (conceptual self, being the basis of standpoints and views) cannot be found when the mind has clarity.

Plato

See also: Plato and Theory of forms

Plato (429–347 B.C.E.) believed that the material world is a shadow of a higher reality that consists of concepts he called Forms. According to Plato, objects in our everyday world "participate in" these Forms, which confer identity and meaning to material objects. For example, a circle drawn in the sand would be a circle only because it participates in the concept of an ideal circle that exists somewhere in the world of Forms. He argued that, as the body is from the material world, the soul is from the world of Forms and is thus immortal. He believed the soul was temporarily united with the body and would only be separated at death, when it, if pure, would return to the world of Forms; otherwise, reincarnation follows. Since the soul does not exist in time and space, as the body does, it can access universal truths. For Plato, ideas (or Forms) are the true reality, and are experienced by the soul. The body is for Plato empty in that it cannot access the abstract reality of the world; it can only experience shadows. This is determined by Plato's essentially rationalistic epistemology.

Aristotle

Main article: Hylomorphism § Body–soul hylomorphism

For Aristotle (384–322 BC) mind is a faculty of the soul. Regarding the soul, he said:

It is not necessary to ask whether soul and body are one, just as it is not necessary to ask whether the wax and its shape are one, nor generally whether the matter of each thing and that of which it is the matter are one. For even if one and being are spoken of in several ways, what is properly so spoken of is the actuality.

— De Anima ii 1, 412b6–9

In the end, Aristotle saw the relation between soul and body as uncomplicated, in the same way that it is uncomplicated that a cubical shape is a property of a toy building block. The soul is a property exhibited by the body, one among many. Moreover, Aristotle proposed that when the body perishes, so does the soul, just as the shape of a building block disappears with destruction of the block.

Medieval Aristotelianism

Working in the Aristotelian-influenced tradition of Thomism, Thomas Aquinas (1225–1274), like Aristotle, believed that the mind and the body are one, like a seal and wax; therefore, it is pointless to ask whether or not they are one. However, (referring to "mind" as "the soul") he asserted that the soul persists after the death of the body in spite of their unity, calling the soul "this particular thing". Since his view was primarily theological rather than philosophical, it is impossible to fit it neatly within either the category of physicalism or dualism.

Influences of Eastern monotheistic religions

Main articles: Dualistic cosmology and Gnosticism

In religious philosophy of Eastern monotheism, dualism denotes a binary opposition of an idea that contains two essential parts. The first formal concept of a "mind–body" split may be found in the divinity–secularity dualism of the ancient Persian religion of Zoroastrianism around the mid-fifth century BC. Gnosticism is a modern name for a variety of ancient dualistic ideas inspired by Judaism popular in the first and second century AD. These ideas later seem to have been incorporated into Galen's "tripartite soul"  that led into both the Christian sentiments expressed in the later Augustinian theodicy and Avicenna's Platonism in Islamic Philosophy.

Descartes

Main article: René Descartes

René Descartes (1596–1650) believed that mind exerted control over the brain via the pineal gland:

My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed.

— René Descartes, Treatise of Man

[The] mechanism of our body is so constructed that simply by this gland's being moved in any way by the soul or by any other cause, it drives the surrounding spirits towards the pores of the brain, which direct them through the nerves to the muscles; and in this way the gland makes the spirits move the limbs.

— René Descartes, Passions of the Soul

His posited relation between mind and body is called Cartesian dualism or substance dualism. He held that mind was distinct from matter, but could influence matter. How such an interaction could be exerted remains a contentious issue.

Kant

Main article: Immanuel Kant

For Immanuel Kant (1724–1804) beyond mind and matter there exists a world of a priori forms, which are seen as necessary preconditions for understanding. Some of these forms, space and time being examples, today seem to be pre-programmed in the brain.

...whatever it is that impinges on us from the mind-independent world does not come located in a spatial or a temporal matrix,...The mind has two pure forms of intuition built into it to allow it to... organize this 'manifold of raw intuition'.

— Andrew Brook, Kant's view of the mind and consciousness of self: Transcendental aesthetic

Kant views the mind–body interaction as taking place through forces that may be of different kinds for mind and body.

Huxley

Main article: Thomas Henry Huxley

For Thomas Henry Huxley (1825–1895) the conscious mind was a by-product of the brain that has no influence upon the brain, a so-called epiphenomenon.

On the epiphenomenalist view, mental events play no causal role. Huxley, who held the view, compared mental events to a steam whistle that contributes nothing to the work of a locomotive.

— William Robinson, Epiphenomenalism

Whitehead

Main article: Alfred North Whitehead

Alfred North Whitehead advocated a sophisticated form of panpsychism that has been called by David Ray Griffin panexperientialism.

Popper

Main article: Karl Popper

For Karl Popper (1902–1994) there are three aspects of the mind–body problem: the worlds of matter, mind, and of the creations of the mind, such as mathematics. In his view, the third-world creations of the mind could be interpreted by the second-world mind and used to affect the first-world of matter. An example might be radio, an example of the interpretation of the third-world (Maxwell's electromagnetic theory) by the second-world mind to suggest modifications of the external first world.

The body–mind problem is the question of whether and how our thought processes in World 2 are bound up with brain events in World 1. ...I would argue that the first and oldest of these attempted solutions is the only one that deserves to be taken seriously [namely]: World 2 and World 1 interact, so that when someone reads a book or listens to a lecture, brain events occur that act upon the World 2 of the reader's or listener's thoughts; and conversely, when a mathematician follows a proof, his World 2 acts upon his brain and thus upon World 1. This, then, is the thesis of body–mind interaction.

— Karl Popper, Notes of a realist on the body–mind problem

Ryle

Main article: Gilbert Ryle

With his 1949 book, The Concept of Mind, Gilbert Ryle "was seen to have put the final nail in the coffin of Cartesian dualism".

In the chapter "Descartes' Myth," Ryle introduces "the dogma of the Ghost in the machine" to describe the philosophical concept of the mind as an entity separate from the body:

I hope to prove that it is entirely false, and false not in detail but in principle. It is not merely an assemblage of particular mistakes. It is one big mistake and a mistake of a special kind. It is, namely, a category mistake.

Searle

Main article: John Searle

For John Searle (1932-2025) the mind–body problem is a false dichotomy; that is, mind is a perfectly ordinary aspect of the brain. Searle proposed biological naturalism in 1980.

According to Searle then, there is no more a mind–body problem than there is a macro–micro economics problem. They are different levels of description of the same set of phenomena. [...] But Searle is careful to maintain that the mental – the domain of qualitative experience and understanding – is autonomous and has no counterpart on the microlevel; any redescription of these macroscopic features amounts to a kind of evisceration, ...

— Joshua Rust, John Searle

Nature versus nurture

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Nature_versus_nurture

Nature versus nurture is a long-standing debate in biology and society about the relative influence on human behavior of their genetic inheritance or biology (nature) and the environmental conditions of their development (nurture). Nature is what people think of as pre-wiring and is influenced by genetic inheritance and other biological factors. Nurture is generally taken as the influence of external factors after conception e.g. the product of exposure, experience, and learning on an individual.

The phrase in its modern sense was popularized by the Victorian polymath Francis Galton, the modern founder of eugenics and behavioral genetics when he was discussing the influence of heredity and environment on social advancement. Galton was influenced by On the Origin of Species written by his half-cousin, the evolutionary biologist Charles Darwin. The complementary combination of the two concepts is an ancient concept (Ancient Greek: ἁπό φύσεως καὶ εὐτροφίας).

The view that humans acquire all or almost all their behavioral traits from "nurture" was termed tabula rasa ('blank tablet, slate') by John Locke in 1690. A blank slate view (sometimes termed blank-slatism) in human developmental psychology, which assumes that human behavioral traits develop almost exclusively from environmental influences, was widely held during much of the 20th century. The debate between "blank-slate" denial of the influence of heritability, and the view admitting both environmental and heritable traits, was at the core of an ideological dispute over research agendas throughout the second half of the 20th century.

Today, the idea of a strong dichotomy of nature versus nurture is generally considered to have limited relevance. Both "nature" and "nurture" factors have been found to contribute substantially, often in an inextricable manner, to many processes studied by biologists, psychologists and anthropologists. Feedback loops have been found in which nature and nurture influence one another constantly, as seen in self-domestication. In ecology and behavioral genetics, researchers posit that nurture has an essential influence on the nature of an individual. Similarly in other fields, the dividing line between an inherited and an acquired trait has become unclear, as in epigenetics and fetal development.

History of the debate

According to Records of the Grand Historian (94 BC) by Sima Qian, during Chen Sheng Wu Guang uprising in 209 B.C., Chen Sheng asked the rhetorical question as a call to war: "Are kings, generals, and ministers merely born into their kind?" (Chinese: 王侯將相寧有種乎). Though Chen was obviously negative to the question, the phrase has often been cited as an early quest into the nature versus nurture problem.

John Locke's An Essay Concerning Human Understanding (1690) is often cited as the foundational document of the blank slate view. In the Essay, Locke specifically criticizes René Descartes's claim of an innate idea of God that is universal to humanity. Locke's view was harshly criticized in his own time. Anthony Ashley-Cooper, 3rd Earl of Shaftesbury, complained that by denying the possibility of any innate ideas, Locke "threw all order and virtue out of the world," leading to total moral relativism. By the 19th century, the predominant perspective was contrary to that of Locke's, tending to focus on "instinct." Leda Cosmides and John Tooby noted that William James (1842–1910) argued that humans have more instincts than animals, and that greater freedom of action is the result of having more psychological instincts, not fewer.

The question of "innate ideas" or "instincts" was of some importance in the discussion of free will in moral philosophy. In 18th-century philosophy, this was cast in terms of "innate ideas" establishing the presence of a universal virtue, a prerequisite for objective morals. In the 20th century, this argument was in a way inverted, since some philosophers (J. L. Mackie) now argued that the evolutionary origins of human behavioral traits forces us to concede that there is no foundation for ethics, while others (Thomas Nagel) treated ethics as a field of cognitively valid statements in complete isolation from evolutionary considerations.

Early to mid-20th century

In the early 20th century, there was an increased interest in the role of one's environment, as a reaction to the strong focus on pure heredity in the wake of the triumphal success of Darwin's theory of evolution. During this time, the social sciences developed as the project of studying the influence of culture in clean isolation from questions related to "biology. Franz Boas's The Mind of Primitive Man (1911) established a program that would dominate American anthropology for the next 15 years. In this study, he established that in any given population, biology, language, material, and symbolic culture, are autonomous; that each is an equally important dimension of human nature, but that none of these dimensions is reducible to another.

Purist behaviorism

John B. Watson in the 1920s and 1930s established the school of purist behaviorism that would become dominant over the following decades. Watson is often said to have been convinced of the complete dominance of cultural influence over anything that heredity might contribute. This is based on the following quote which is frequently repeated without context, as the last sentence is frequently omitted, leading to confusion about Watson's position:

Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select – doctor, lawyer, artist, merchant-chief and, yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors. I am going beyond my facts and I admit it, but so have the advocates of the contrary and they have been doing it for many thousands of years.

During the 1940s to 1960s, Ashley Montagu was a notable proponent of this purist form of behaviorism which allowed no contribution from heredity whatsoever:  an is man because he has no instincts, because everything he is and has become he has learned, acquired, from his culture ... with the exception of the instinctoid reactions in infants to sudden withdrawals of support and to sudden loud noises, the human being is entirely instinctless.

In 1951, Calvin Hall suggested that the dichotomy opposing nature to nurture is ultimately fruitless.

In African Genesis (1961) and The Territorial Imperative (1966), Robert Ardrey argues for innate attributes of human nature, especially concerning territoriality. Desmond Morris in The Naked Ape (1967) expresses similar views. Organised opposition to Montagu's kind of purist "blank-slatism" began to pick up in the 1970s, notably led by E. O. Wilson (On Human Nature, 1979).

The tool of twin studies was developed as a research design intended to exclude all confounders based on inherited behavioral traits. Such studies are designed to decompose the variability of a given trait in a given population into a genetic and an environmental component. Studies using twin neuroimaging methods show that genetic elements explain more of the variation in cognitive processing than emotional regulation which indicates that environmental factors play a greater role in shaping affective traits. Twin studies established that there was, in many cases, a significant heritable component. These results did not, in any way, point to overwhelming contribution of heritable factors, with heritability typically ranging around 40% to 50%, so that the controversy may not be cast in terms of purist behaviorism vs. purist nativism. Rather, it was purist behaviorism that was gradually replaced by the now-predominant view that both kinds of factors usually contribute to a given trait, anecdotally phrased by Donald Hebb as an answer to the question "which, nature or nurture, contributes more to personality?" by asking in response, "Which contributes more to the area of a rectangle, its length or its width?"

In a comparable avenue of research, anthropologist Donald Brown in the 1980s surveyed hundreds of anthropological studies from around the world and collected a set of cultural universals. He identified approximately 150 such features, coming to the conclusion there is indeed a "universal human nature", and that these features point to what that universal human nature is.

Determinism

See also: Social determinism, Cultural determinism, and Biological determinism

At the height of the controversy, during the 1970s to 1980s, the debate was highly ideologised. In Not in Our Genes: Biology, Ideology and Human Nature (1984), Richard Lewontin, Steven Rose and Leon Kamin criticise "genetic determinism" from a Marxist framework, arguing that "Science is the ultimate legitimator of bourgeois ideology ... If biological determinism is a weapon in the struggle between classes, then the universities are weapons factories, and their teaching and research faculties are the engineers, designers, and production workers." The debate thus shifted away from whether heritable traits exist to whether it was politically or ethically permissible to admit their existence. The authors deny this, requesting that evolutionary inclinations be discarded in ethical and political discussions regardless of whether they exist or not.

1990s

Heritability studies became much easier to perform, and hence much more numerous, with the advances of genetic studies during the 1990s. By the late 1990s, an overwhelming amount of evidence had accumulated that amounts to a refutation of the extreme forms of "blank-slatism" advocated by Watson or Montagu.

This revised state of affairs was summarized in books aimed at a popular audience from the late 1990s. In The Nurture Assumption: Why Children Turn Out the Way They Do (1998), Judith Rich Harris was heralded by Steven Pinker as a book that "will come to be seen as a turning point in the history of psychology." However, Harris was criticized for exaggerating the point of "parental upbringing seems to matter less than previously thought" to the implication that "parents do not matter."

The situation as it presented itself by the end of the 20th century was summarized in The Blank Slate: The Modern Denial of Human Nature (2002) by Steven Pinker. The book became a best-seller, and was instrumental in bringing to the attention of a wider public the paradigm shift away from the behaviourist purism of the 1940s to 1970s that had taken place over the preceding decades.

Pinker portrays the adherence to pure blank-slatism as an ideological dogma linked to two other dogmas found in the dominant view of human nature in the 20th century:

1. "noble savage", in the sense that people are born good and corrupted by bad influence; and
2. "ghost in the machine", in the sense that there is a human soul capable of moral choices completely detached from biology.

Pinker argues that all three dogmas were held onto for an extended period even in the face of evidence because they were seen as desirable in the sense that if any human trait is purely conditioned by culture, any undesired trait (such as crime or aggression) may be engineered away by purely cultural (political) means. Pinker focuses on reasons he assumes were responsible for unduly repressing evidence to the contrary, notably the fear of (imagined or projected) political or ideological consequences.

Heritability estimates

Main article: Heritability

This chart illustrates three patterns one might see when studying the influence of genes and environment on traits in individuals. Trait A shows a high sibling correlation, but little heritability (i.e. high shared environmental variance c²; low heritability h²). Trait B shows a high heritability since the correlation of trait rises sharply with the degree of genetic similarity. Trait C shows low heritability, but also low correlations generally; this means Trait C has a high nonshared environmental variance e². In other words, the degree to which individuals display Trait C has little to do with either genes or broadly predictable environmental factors—roughly, the outcome approaches random for an individual. Notice also that even identical twins who are raised in a common family rarely show 100% trait correlation.

The term heritability only refers to the degree of genetic variation between people on a trait. It does not refer to the degree to which a trait of a particular individual is due to environmental or genetic factors. The traits of an individual are always a complex interweaving of both. For an individual, even strongly genetically influenced, or "obligate" traits, such as eye color, assume the inputs of a typical environment during ontogenetic development (e.g., certain ranges of temperatures, oxygen levels, etc.).

In contrast, the "heritability index" statistically quantifies the extent to which variation between individuals on a trait is due to variation in the genes those individuals carry. In animals where breeding and environments can be controlled experimentally, heritability can be determined relatively easily. Such experiments would be unethical for human research. This problem can be overcome by finding existing populations of humans that reflect the experimental setting the researcher wishes to create.

One way to determine the contribution of genes and environment to a trait is to study twins. In one kind of study, identical twins reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. Twins reared apart are not assigned at random to foster or adoptive parents. In another kind of twin study, identical twins reared together (who share family environment and genes) are compared to fraternal twins reared together (who also share family environment but only share half their genes). Another condition that permits the disassociation of genes and environment is adoption. In one kind of adoption study, biological siblings reared together (who share the same family environment and half their genes) are compared to adoptive siblings (who share their family environment but none of their genes).

In many cases, it has been found that genes make a substantial contribution, including psychological traits such as intelligence and personality. Yet heritability may differ in other circumstances, for instance environmental deprivation. Examples of low, medium, and high heritability traits include:

Low heritability	Medium heritability	High heritability
Specific language	Weight	Blood type
Specific religion	Religiosity	Eye color

Twin and adoption studies have their methodological limits. For example, both are limited to the range of environments and genes which they sample. Almost all of these studies are conducted in Western countries, and therefore cannot necessarily be extrapolated globally to include non-western populations. Additionally, both types of studies depend on particular assumptions, such as the equal environments assumption in the case of twin studies, and the lack of pre-adoptive effects in the case of adoption studies.

Since the definition of "nature" in this context is tied to "heritability", the definition of "nurture" has consequently become very wide, including any type of causality that is not heritable. The term has thus moved away from its original connotation of "cultural influences" to include all effects of the environment, including; indeed, a substantial source of environmental input to human nature may arise from stochastic variations in prenatal development and is thus in no sense of the term "cultural".

Gene–environment interaction

Main article: Gene–environment interaction

Many properties of the brain are genetically organized, and don't depend on information coming in from the senses.

— Steven Pinker

The interactions of genes with environment, called gene–environment interactions, are another component of the nature–nurture debate. A classic example of gene–environment interaction is the ability of a diet low in the amino acid phenylalanine to partially suppress the genetic disease phenylketonuria. Yet another complication to the nature–nurture debate is the existence of gene–environment correlations. These correlations indicate that individuals with certain genotypes are more likely to find themselves in certain environments. Thus, it appears that genes can shape (the selection or creation of) environments. Even using experiments like those described above, it can be very difficult to determine convincingly the relative contribution of genes and environment. The analogy "genetics loads the gun, but environment pulls the trigger" has been attributed to Judith Stern.

Heritability refers to the origins of differences between people. Individual development, even of highly heritable traits, such as eye color, depends on a range of environmental factors, from the other genes in the organism, to physical variables such as temperature, oxygen levels etc. during its development or ontogenesis.

The variability of trait can be meaningfully spoken of as being due in certain proportions to genetic differences ("nature"), or environments ("nurture"). For highly penetrant Mendelian genetic disorders such as Huntington's disease virtually all the incidence of the disease is due to genetic differences. Huntington's animal models live much longer or shorter lives depending on how they are cared for.

At a molecular level, genes interact with signals from other genes and from the environment. While there are many thousands of single-gene-locus traits, so-called complex traits are due to the additive effects of many (often hundreds) of small gene effects. A good example of this is height, where variance appears to be spread across many hundreds of loci.

Extreme genetic or environmental conditions can predominate in rare circumstances—if a child is born mute due to a genetic mutation, it will not learn to speak any language regardless of the environment; similarly, someone who is practically certain to eventually develop Huntington's disease according to their genotype may die in an unrelated accident (an environmental event) long before the disease will manifest itself.

The "two buckets" view of heritability

More realistic "homogenous mudpie" view of heritability

Steven Pinker likewise described several examples:

[C]oncrete behavioral traits that patently depend on content provided by the home or culture—which language one speaks, which religion one practices, which political party one supports—are not heritable at all. But traits that reflect the underlying talents and temperaments—how proficient with language a person is, how religious, how liberal or conservative—are partially heritable.

When traits are determined by a complex interaction of genotype and environment it is possible to measure the heritability of a trait within a population. However, many non-scientists who encounter a report of a trait having a certain percentage heritability imagine non-interactional, additive contributions of genes and environment to the trait. As an analogy, some laypeople may think of the degree of a trait being made up of two "buckets," genes and environment, each able to hold a certain capacity of the trait. But even for intermediate heritabilities, a trait is always shaped by both genetic dispositions and the environments in which people develop, merely with greater and lesser plasticities associated with these heritability measures.

Heritability measures always refer to the degree of variation between individuals in a population. That is, as these statistics cannot be applied at the level of the individual, it would be incorrect to say that while the heritability index of personality is about 0.6, 60% of one's personality is obtained from one's parents and 40% from the environment. To help to understand this, imagine that all humans were genetic clones. The heritability index for all traits would be zero (all variability between clonal individuals must be due to environmental factors). And, contrary to erroneous interpretations of the heritability index, as societies become more egalitarian (everyone has more similar experiences) the heritability index goes up (as environments become more similar, variability between individuals is due more to genetic factors).

One should also take into account the fact that the variables of heritability and environmentality are not precise and vary within a chosen population and across cultures. It would be more accurate to state that the degree of heritability and environmentality is measured in its reference to a particular phenotype in a chosen group of a population in a given period of time. The accuracy of the calculations is further hindered by the number of coefficients taken into consideration, age being one such variable. The display of the influence of heritability and environmentality differs drastically across age groups: the older the studied age is, the more noticeable the heritability factor becomes, the younger the test subjects are, the more likely it is to show signs of strong influence of the environmental factors.

For example, one study found no statistically significant difference in self-reported wellbeing between middle-aged monozygotic twins separated at birth and those reared in the same household, suggesting that happiness in middle-aged adults is not based in environmental factors related to family rearing. The same result was also found among middle-aged dizygotic twins. Furthermore, there was significantly more variance in the dizygotic twins' self-reported wellbeing than there was in the monozygotic group. Genetic similarity has thus been estimated to account for around 50% of the variance in adult happiness at a given point in time, and as much as 80% of the variance in long-term happiness stability. Other studies have similarly found the heritability of happiness to be around 0.35–0.50.

Some have pointed out that environmental inputs affect the expression of genes. This is one explanation of how environment can influence the extent to which a genetic disposition will actually manifest.

Obligate vs. facultative adaptations

Traits may be considered to be adaptations (such as the umbilical cord), byproducts of adaptations (the belly button) or due to random variation (convex or concave belly button shape). An alternative to contrasting nature and nurture focuses on "obligate vs. facultative" adaptations. Adaptations may be generally more obligate (robust in the face of typical environmental variation) or more facultative (sensitive to typical environmental variation). For example, the rewarding sweet taste of sugar and the pain of bodily injury are obligate psychological adaptations—typical environmental variability during development does not much affect their operation.

On the other hand, facultative adaptations are somewhat like "if-then" statements. An example of a facultative psychological adaptation may be adult attachment style. The attachment style of adults, (for example, a "secure attachment style," the propensity to develop close, trusting bonds with others) is proposed to be conditional on whether an individual's early childhood caregivers could be trusted to provide reliable assistance and attention. An example of a facultative physiological adaptation is tanning of skin on exposure to sunlight (to prevent skin damage). Facultative social adaptation have also been proposed. For example, whether a society is warlike or peaceful has been proposed to be conditional on how much collective threat that society is experiencing.

Advanced techniques

Quantitative studies of heritable traits throw light on the question.

Developmental genetic analysis examines the effects of genes over the course of a human lifespan. Early studies of intelligence, which mostly examined young children, found that heritability measured 40–50%. Subsequent developmental genetic analyses found that variance attributable to additive environmental effects is less apparent in older individuals, with estimated heritability of IQ increasing in adulthood.

Multivariate genetic analysis examines the genetic contribution to several traits that vary together. For example, multivariate genetic analysis has demonstrated that the genetic determinants of all specific cognitive abilities (e.g., memory, spatial reasoning, processing speed) overlap greatly, such that the genes associated with any specific cognitive ability will affect all others. Similarly, multivariate genetic analysis has found that genes that affect scholastic achievement completely overlap with the genes that affect cognitive ability.

Extremes analysis examines the link between normal and pathological traits. For example, it is hypothesized that a given behavioral disorder may represent an extreme of a continuous distribution of a normal behavior and hence an extreme of a continuous distribution of genetic and environmental variation. Depression, phobias, and reading disabilities have been examined in this context.

For a few highly heritable traits, studies have identified loci associated with variance in that trait, for instance in some individuals with schizophrenia. New research is showing that gene expression can happen in adults due to environmental stimuli. For example, people with schizophrenic gene have a genetic predisposition for this illness but the gene lays dormant in most people. However, if introduced to chronic stress or introducing some amphetamines it caused the methyl groups to stick to hippocampi histones.

Intelligence

Heritability of intelligence

Main article: Heritability of IQ

Cognitive functions have a significant genetic component. A 2015 meta-analysis of over 14 million twin pairs found that genetics explained 57% of the variability in cognitive functions. Evidence from behavioral genetic research suggests that family environmental factors may have an effect upon childhood IQ, accounting for up to a quarter of the variance. The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1995) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Here, environment is playing a role in what is believed to be fully genetic (intelligence) but it was found that severely deprived, neglectful, or abusive environments have highly negative effects on many aspects of children's intellect development. Beyond that minimum, however, the role of family experience is in serious dispute. On the other hand, by late adolescence this correlation disappears, such that adoptive siblings no longer have similar IQ scores.

Moreover, adoption studies indicate that, by adulthood, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.74), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (≈0.0). Recent adoption studies also found that supportive parents can have a positive effect on the development of their children.

Environment's effect on IQ

Other studies have focused on environmental factors that may affect IQ. For example, research has shown that factors such as access to education, nutrition, and social support can have a significant impact on IQ. Recent research shows that children who spend time in nature develop better executive functioning and attention skills according to recent meta-analytic studies although the results show small effects that vary between studies. A study examining the brain-wide associations of hundreds of environmental and phenotypic factors showed socioeconomic status and IQ to have similar brain patterns linked to arousal, sleep deprivation and stress, with socioeconomic status having the stronger brain links, relative to IQ.

Personality traits

Main article: Personality psychology § Genetic basis of personality

Personality is a frequently cited example of a heritable trait that has been studied in twins and adoptees using behavioral genetic study designs. The most famous categorical organization of heritable personality traits were defined in the 1970s by two research teams led by Paul Costa & Robert R. McCrae and Warren Norman & Lewis Goldberg in which they had people rate their personalities on 1000+ dimensions they then narrowed these down into "The Big Five" factors of personality—openness, conscientiousness, extraversion, agreeableness, and neuroticism. Studies have found that extraversion has a genetic component, with estimates of heritability ranging from 30% to 50%. The close genetic relationship between positive personality traits and, for example, our happiness traits are the mirror images of comorbidity in psychopathology. These personality factors were consistent across cultures, and many studies have also tested the heritability of these traits. Personal agency also factors into this debate. While genetic and environmental factors can shape personality, individuals also have agency in shaping their own personality through their choices, behaviors, and attitudes. For example, one study found that college students who participated in study abroad programs scored higher on measures of openness to experience compared to those who did not participate. Another study found that individuals who lived in diverse neighborhoods were more likely to score higher on openness to experience compared to those who lived in more homogenous neighborhoods.

Identical twins reared apart are far more similar in personality than randomly selected pairs of people. Likewise, identical twins are more similar than fraternal twins. Also, biological siblings are more similar in personality than adoptive siblings. Each observation suggests that personality is heritable to a certain extent. A supporting article had focused on the heritability of personality (which is estimated to be around 50% for subjective well-being) in which a study was conducted using a representative sample of 973 twin pairs to test the heritable differences in subjective well-being which were found to be fully accounted for by the genetic model of the Five-Factor Model's personality domains. However, these same study designs allow for the examination of environment as well as genes.

Adoption studies also directly measure the strength of shared family effects. Adopted siblings share only family environment. Most adoption studies indicate that by adulthood the personalities of adopted siblings are little or no more similar than random pairs of strangers. This would mean that shared family effects on personality are zero by adulthood.

In the case of personality traits, non-shared environmental effects are often found to out-weigh shared environmental effects. That is, environmental effects that are typically thought to be life-shaping (such as family life) may have less of an impact than non-shared effects, which are harder to identify. One possible source of non-shared effects is the environment of pre-natal development. Random variations in the genetic program of development may be a substantial source of non-shared environment. These results suggest that "nurture" may not be the predominant factor in "environment". Environment and our situations, do in fact impact our lives, but not the way in which we would typically react to these environmental factors. We are preset with personality traits that are the basis for how we would react to situations. An example would be how extraverted prisoners become less happy than introverted prisoners and would react to their incarceration more negatively due to their preset extraverted personality. Behavioral genes are somewhat proven to exist when we take a look at fraternal twins. When fraternal twins are reared apart, they show the same similarities in behavior and response as if they have been reared together.

Genetics

The relationship between personality and people's own well-being is influenced and mediated by genes. There has been found to be a stable set point for happiness that is characteristic of the individual (largely determined by the individual's genes). Happiness fluctuates around that setpoint (again, genetically determined) based on whether good things or bad things are happening to us ("nurture"), but only fluctuates in small magnitude in a normal human. The midpoint of these fluctuations is determined by the "great genetic lottery" that people are born with, which leads them to conclude that how happy they may feel at the moment or over time is simply due to the luck of the draw, or gene. This fluctuation was also not due to educational attainment, which only accounted for less than 2% of the variance in well-being for women, and less than 1% of the variance for men.

They consider that the individualities measured together with personality tests remain steady throughout an individual's lifespan. They further believe that human beings may refine their forms or personality but can never change them entirely. Darwin's Theory of Evolution steered naturalists such as George Williams and William Hamilton to the concept of personality evolution. They suggested that physical organs and also personality is a product of natural selection.

With the advent of gene sequencing, it has become possible to search for and identify specific gene polymorphisms that affect traits such as IQ and personality. These techniques work by tracking the association of differences in a trait of interest with differences in specific molecular markers or functional variants.

In contrast to views developed in 1960s that gender identity is primarily learned (which led to a protocol of surgical sex changes in male infants with injured or malformed genitals, such as David Reimer), genomics has provided solid evidence that both sex and gender identities are primarily influenced by genes:

It is now clear that genes are vastly more influential than virtually any other force in shaping sex identity and gender identity ... The growing consensus in medicine is that ... children should be assigned to their chromosomal (i.e., genetic) sex regardless of anatomical variations and differences—with the option of switching, if desired, later in life.

— Siddhartha Mukherjee, The Gene: An Intimate History, 2016