Nature versus nurture

Nature versus nurture

Overview
The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism
Psychological nativism
In the field of psychology, nativism is the view that certain skills or abilities are 'native' or hard wired into the brain at birth. This is in contrast to empiricism, the 'blank slate' or tabula rasa view, which states that the brain has inborn capabilities for learning from the environment but...

, or innatism
Innatism
Innatism is a philosophical doctrine that holds that the mind is born with ideas/knowledge, and that therefore the mind is not a 'blank slate' at birth, as early empiricists such as John Locke claimed...

) versus personal experiences ("nurture," i.e. empiricism
Empiricism
Empiricism is a theory of knowledge that asserts that knowledge comes only or primarily via sensory experience. One of several views of epistemology, the study of human knowledge, along with rationalism, idealism and historicism, empiricism emphasizes the role of experience and evidence,...

 or behaviorism
Behaviorism
Behaviorism , also called the learning perspective , is a philosophy of psychology based on the proposition that all things that organisms do—including acting, thinking, and feeling—can and should be regarded as behaviors, and that psychological disorders are best treated by altering behavior...

) in determining
Determinism
Determinism is the general philosophical thesis that states that for everything that happens there are conditions such that, given them, nothing else could happen. There are many versions of this thesis. Each of them rests upon various alleged connections, and interdependencies of things and...

 or causing
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

 individual differences in physical
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 and behavior
Behavior
Behavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...

al traits.

The phrase "Nature versus nurture" in its modern sense was coined by the English
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 Victorian
Victorian era
The Victorian era of British history was the period of Queen Victoria's reign from 20 June 1837 until her death on 22 January 1901. It was a long period of peace, prosperity, refined sensibilities and national self-confidence...

 polymath
Polymath
A polymath is a person whose expertise spans a significant number of different subject areas. In less formal terms, a polymath may simply be someone who is very knowledgeable...

 Francis Galton
Francis Galton
Sir Francis Galton /ˈfrɑːnsɪs ˈgɔːltn̩/ FRS , cousin of Douglas Strutt Galton, half-cousin of Charles Darwin, was an English Victorian polymath: anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician...

 in discussion of the influence of heredity
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...

 and environment
Environment (biophysical)
The biophysical environment is the combined modeling of the physical environment and the biological life forms within the environment, and includes all variables, parameters as well as conditions and modes inside the Earth's biosphere. The biophysical environment can be divided into two categories:...

 on social advancement, although the terms had been contrasted previously, for example by Shakespeare
William Shakespeare
William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

 (in his play, The Tempest
The Tempest
The Tempest is a play by William Shakespeare, believed to have been written in 1610–11, and thought by many critics to be the last play that Shakespeare wrote alone. It is set on a remote island, where Prospero, the exiled Duke of Milan, plots to restore his daughter Miranda to her rightful place,...

: 4.1).
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Encyclopedia
The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism
Psychological nativism
In the field of psychology, nativism is the view that certain skills or abilities are 'native' or hard wired into the brain at birth. This is in contrast to empiricism, the 'blank slate' or tabula rasa view, which states that the brain has inborn capabilities for learning from the environment but...

, or innatism
Innatism
Innatism is a philosophical doctrine that holds that the mind is born with ideas/knowledge, and that therefore the mind is not a 'blank slate' at birth, as early empiricists such as John Locke claimed...

) versus personal experiences ("nurture," i.e. empiricism
Empiricism
Empiricism is a theory of knowledge that asserts that knowledge comes only or primarily via sensory experience. One of several views of epistemology, the study of human knowledge, along with rationalism, idealism and historicism, empiricism emphasizes the role of experience and evidence,...

 or behaviorism
Behaviorism
Behaviorism , also called the learning perspective , is a philosophy of psychology based on the proposition that all things that organisms do—including acting, thinking, and feeling—can and should be regarded as behaviors, and that psychological disorders are best treated by altering behavior...

) in determining
Determinism
Determinism is the general philosophical thesis that states that for everything that happens there are conditions such that, given them, nothing else could happen. There are many versions of this thesis. Each of them rests upon various alleged connections, and interdependencies of things and...

 or causing
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

 individual differences in physical
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 and behavior
Behavior
Behavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...

al traits.

The phrase "Nature versus nurture" in its modern sense was coined by the English
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 Victorian
Victorian era
The Victorian era of British history was the period of Queen Victoria's reign from 20 June 1837 until her death on 22 January 1901. It was a long period of peace, prosperity, refined sensibilities and national self-confidence...

 polymath
Polymath
A polymath is a person whose expertise spans a significant number of different subject areas. In less formal terms, a polymath may simply be someone who is very knowledgeable...

 Francis Galton
Francis Galton
Sir Francis Galton /ˈfrɑːnsɪs ˈgɔːltn̩/ FRS , cousin of Douglas Strutt Galton, half-cousin of Charles Darwin, was an English Victorian polymath: anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician...

 in discussion of the influence of heredity
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...

 and environment
Environment (biophysical)
The biophysical environment is the combined modeling of the physical environment and the biological life forms within the environment, and includes all variables, parameters as well as conditions and modes inside the Earth's biosphere. The biophysical environment can be divided into two categories:...

 on social advancement, although the terms had been contrasted previously, for example by Shakespeare
William Shakespeare
William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

 (in his play, The Tempest
The Tempest
The Tempest is a play by William Shakespeare, believed to have been written in 1610–11, and thought by many critics to be the last play that Shakespeare wrote alone. It is set on a remote island, where Prospero, the exiled Duke of Milan, plots to restore his daughter Miranda to her rightful place,...

: 4.1). Galton was influenced by the book On the Origin of Species written by his cousin, Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

. The concept embodied in the phrase has been criticized for its binary simplification of two tightly interwoven parameters, as for example an environment of wealth, education and social privilege are often historically passed to genetic offspring.

The view that humans acquire all or almost all their behavioral traits from "nurture" was termed by philosopher John Locke
John Locke
John Locke FRS , widely known as the Father of Liberalism, was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, he is equally important to social...

 tabula rasa
Tabula rasa
Tabula rasa is the epistemological theory that individuals are born without built-in mental content and that their knowledge comes from experience and perception. Generally proponents of the tabula rasa thesis favour the "nurture" side of the nature versus nurture debate, when it comes to aspects...

("blank slate") and proposes that humans develop from only environmental influences. This question was once considered to be an appropriate division of developmental influences, but since both types of factors are known to play such interacting roles in development, most modern psychologists and anthropologists consider the question naive—representing an outdated state of knowledge.

In the social and political sciences
Social sciences
Social science is the field of study concerned with society. "Social science" is commonly used as an umbrella term to refer to a plurality of fields outside of the natural sciences usually exclusive of the administrative or managerial sciences...

, the nature versus nurture debate may be contrasted with the structure versus agency
Structure and agency
The question over the primacy of either structure or agency in human behavior is a central debate in the social sciences. In this context, "agency" refers to the capacity of individuals to act independently and to make their own free choices. "Structure", by contrast, refers to the recurrent...

 debate (i.e. socialization
Socialization
Socialization is a term used by sociologists, social psychologists, anthropologists, political scientists and educationalists to refer to the process of inheriting and disseminating norms, customs and ideologies...

 versus individual autonomy). For a discussion of nature versus nurture in language
Language
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

 and other human universals
Cultural universal
A cultural universal , as discussed by George Murdock, Claude Lévi-Strauss, Donald Brown and others, is an element, pattern, trait, or institution that is common to all human cultures worldwide. Taken together, the whole body of cultural universals is known as the human condition...

, see also psychological nativism
Psychological nativism
In the field of psychology, nativism is the view that certain skills or abilities are 'native' or hard wired into the brain at birth. This is in contrast to empiricism, the 'blank slate' or tabula rasa view, which states that the brain has inborn capabilities for learning from the environment but...

.

Scientific approach


To disentangle the effects of genes and environment, behavioral geneticists
Behavioural genetics
Quantitative human behavioural genetics is a specialisation in the biological field of behaviour genetics that studies the role of genetics in human behaviour employing quantitative-genetic methods. The field is an overlap of quantitative genetics and psychology...

 perform adoption
Adoption
Adoption is a process whereby a person assumes the parenting for another and, in so doing, permanently transfers all rights and responsibilities from the original parent or parents...

 and twin studies
Twin study
Twin studies help disentangle the relative importance of environmental and genetic influences on individual traits and behaviors. Twin research is considered a key tool in behavioral genetics and related fields...

. These seek to decompose the variance in a population into genetic and environmental components. This move from individuals to populations makes a critical difference in the way we think about nature and nurture. This difference is perhaps highlighted in the quote attributed to psychologist Donald Hebb who is said to have once answered a journalist's question of "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?" For a particular rectangle, its area is indeed the product of its length and width. Moving to a population, however, this analogy masks the fact that there are many individuals, and that it is meaningful to talk about their differences. Thus if a game such as soccer defined the width of a playing field very tightly, but left the length unspecified, then differences in the area of the playing fields would be almost entirely due to differences in length.

Scientific approaches also seek to break down variance beyond these two categories of nature and nurture. Thus rather than "nurture", behavior geneticists distinguish shared family factors (i.e., those shared by siblings, making them more similar) and nonshared factors (i.e., those that uniquely affect individuals, making siblings different). To express the portion of the variance due to the "nature" component, behavioral geneticists generally refer to the heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...

 of a trait.

With regard to the Big Five personality traits
Big Five personality traits
In contemporary psychology, the "Big Five" factors of personality are five broad domains or dimensions of personality which are used to describe human personality....

 as well as adult IQ in the general U.S. population, the portion of the overall variance that can be attributed to shared family effects is often negligible. On the other hand, most traits are thought to be at least partially heritable. In this context, the "nature" component of the variance is generally thought to be more important than that ascribed to the influence of family upbringing.

In her Pulitzer Prize
Pulitzer Prize
The Pulitzer Prize is a U.S. award for achievements in newspaper and online journalism, literature and musical composition. It was established by American publisher Joseph Pulitzer and is administered by Columbia University in New York City...

-nominated book The Nurture Assumption
The Nurture Assumption
The Nurture Assumption: Why Children Turn Out the Way They Do is a book written by Judith Rich Harris, with a foreword by Steven Pinker, originally published 1998 by the Free Press, which published a revised edition in 2009. It has been published in at least 20 languages...

, author Judith Harris
Judith Rich Harris
Judith Rich Harris is a psychology researcher and the author of The Nurture Assumption, a book criticizing the belief that parents are the most important factor in child development, and presenting evidence which contradicts that belief.Harris has been a resident of Middletown Township, New...

 argues that "nurture," as traditionally defined in terms of family upbringing does not effectively explain the variance for most traits (such as adult IQ and the Big Five personality traits) in the general population of the United States. On the contrary, Harris suggests that either peer groups or random environmental factors (i.e., those that are independent of family upbringing) are more important than family environmental effects.

Although "nurture" has historically been referred to as the care given to children by the parents, with the mother playing a role of particular importance, this term is now regarded by some as any environmental (not genetic) factor in the contemporary nature versus nurture debate. Thus the definition of "nurture" has expanded to include influences on development arising from prenatal, parental, extended family, and peer experiences, and extending to influences such as media, marketing, and socio-economic status. Indeed, a substantial source of environmental input to human nature
Human nature
Human nature refers to the distinguishing characteristics, including ways of thinking, feeling and acting, that humans tend to have naturally....

 may arise from stochastic variations in prenatal development.

Heritability estimates



It is important to note that the term heritability refers only 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
Twin study
Twin studies help disentangle the relative importance of environmental and genetic influences on individual traits and behaviors. Twin research is considered a key tool in behavioral genetics and related fields...

. 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. 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
Adoption
Adoption is a process whereby a person assumes the parenting for another and, in so doing, permanently transfers all rights and responsibilities from the original parent or parents...

. 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 heritabilityMedium heritabilityHigh 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, first-world countries, and therefore cannot be extrapolated globally to include poorer, 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.

Interaction of genes and environment


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
Huntington's disease
Huntington's disease, chorea, or disorder , is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and dementia. It typically becomes noticeable in middle age. HD is the most common genetic cause of abnormal involuntary writhing movements called chorea...

 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
Steven Pinker
Steven Arthur Pinker is a Canadian-American experimental psychologist, cognitive scientist, linguist and popular science author...

 (2004) likewise described several examples:
concrete 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
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

 and environment it is possible to measure the heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...

 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. These statistics cannot be applied at the level of the individual. It is incorrect to say that since the heritability index of personality is about 0.6, you got 60% of your personality from your 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 heritibility 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).

Some have pointed out that environmental inputs affect the expression of genes (see the article on epigenetics
Epigenetics
In biology, and specifically genetics, epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence – hence the name epi- -genetics...

). This is one explanation of how environment can influence the extent to which a genetic disposition will actually manifest. 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
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

 to partially suppress the genetic disease phenylketonuria
Phenylketonuria
Phenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...

. Yet another complication to the nature–nurture debate is the existence of gene-environment correlation
Gene-environment correlation
Gene-environment correlation is said to occur when exposure to environmental conditions depends on an individual's genotype.-Definition:...

s. 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.

Obligate vs. Facultative Adaptations


Traits may be considered likely 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 maybe 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).

Advanced techniques


The power of quantitative studies of heritable traits has been expanded by the development of new techniques. Developmental genetic analysis examines the effects of genes over the course of a human lifespan. For example, early studies of intelligence, which mostly examined young children, found that heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...

 measures 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 being higher than that in adulthood.

Another advanced technique, 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, some studies have identified loci associated with variance in that trait in some individuals. For example, research groups have identified loci that are associated with schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

 (Harrison and Owen, 2003) in subsets of patients with that diagnosis.

IQ debate



Evidence suggests that family environmental factors may have an effect upon childhood IQ
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...

, accounting for up to a quarter of the variance. On the other hand, by late adolescence this correlation disappears, such that adoptive siblings are no more similar in IQ than strangers.

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).