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Debate about heredity and environment as determinants of physical or mental development
Nature versus nurture is a long standing debate in biology and society about the relative influence on human beings of their genetic inheritance (nature) and the environmental conditions of their development (nurture). The alliterative expression "nature and nurture" in English has been in use since at least the Elizabethan period and goes back to medieval French. The complementary combination of the two concepts is an ancient concept (ἁπό φύσεως καὶ εὐτροφίας). 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 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, has often been cast in terms of nature versus nurture. These two conflicting approaches to human development were at the core of an ideological dispute over research agendas throughout the second half of the 20th century. As both "nature" and "nurture" factors were found to contribute substantially, often in an inextricable manner, such views were seen as naive or outdated by most scholars of human development by the 21st century. The strong dichotomy of nature versus nurture has thus been claimed to have limited relevance in some fields of research. Close 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 think 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 becomes unclear, as in epigenetics or fetal development.

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?" (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 to 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" were 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, 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.

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.

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: Man 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. 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.

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.

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 the other extreme, traits such as native language are environmentally determined: linguists have found that any child (if capable of learning a language at all) can learn any human language with equal facility. With virtually all biological and psychological traits, however, genes and environment work in concert, communicating back and forth to create the individual. 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. 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. 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.

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. The budding field of epigenetics has conducted research showing that hereditable conditions like schizophrenia, which have an 80% hereditability with only 10% of those who have inherited the trait actually displaying Schizophrenic traits. 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.

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.

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. Furthermore, research has suggested that certain experiences during early childhood, such as exposure to lead or other environmental toxins, can have a negative impact on IQ. Studies have consistently shown that environmental factors can have a significant impact on IQ. Access to quality education has been found to have a positive effect on IQ, with one study indicating that access to quality preschool education had a lasting impact on IQ scores up to age 35. Malnutrition in early childhood has been linked to lower IQ scores later in life, while supplementation with certain nutrients such as iron and iodine has been shown to improve IQ scores. Social support is also an important environmental factor that positively affects IQ, with one study indicating that children who received high levels of emotional support from their mothers had higher IQ scores than those who received low levels of emotional support.

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%. However, environmental factors such as parenting style, cultural values, and life experiences can also shape a person's level of extraversion. Likewise, neuroticism has a genetic component, with estimates of heritability ranging from 30% to 50%. Environmental factors such as adverse childhood experiences, chronic stress, and cultural values can also influence a person's level of neuroticism. 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.

In their attempts to locate the genes responsible for configuring certain phenotypes, researches resort to two different techniques. Linkage study facilitates the process of determining a specific location in which a gene of interest is located. This methodology is applied only among individuals that are related and does not serve to pinpoint specific genes. It does, however, narrow down the area of search, making it easier to locate one or several genes in the genome which constitute a specific trait. Association studies, on the other hand, are more hypothetic and seek to verify whether a particular genetic variable really influences the phenotype of interest. In association studies it is more common to use case control approach, comparing the subject with relatively higher or lower hereditary determinants with the control subject.