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Nature versus nurture is a long-standing debate in biology and society about the balance between two competing factors which determine fate:
genetics
Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...
(nature) and environment (nurture). The alliterative expression "nature and nurture" in English has been in use since at least the
Elizabethan period
The Elizabethan era is the epoch in the Tudor period of the history of England during the reign of Queen Elizabeth I (1558–1603). Historians often depict it as the golden age in English history. The symbol of Britannia (a female personifi ...
and goes back to
medieval French
Old French (, , ; Modern French: ) was the language spoken in most of the northern half of France from approximately the 8th to the 14th centuries. Rather than a unified language, Old French was a linkage of Romance dialects, mutually intelligi ...
.
The complementary combination of the two concepts is an ancient concept ( grc, 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
A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific pro ...
Francis Galton
Sir Francis Galton, FRS FRAI (; 16 February 1822 – 17 January 1911), was an English Victorian era polymath: a statistician, sociologist, psychologist, anthropologist, tropical explorer, geographer, inventor, meteorologist, proto- ...
, the modern founder of
eugenics
Eugenics ( ; ) is a fringe set of beliefs and practices that aim to improve the genetic quality of a human population. Historically, eugenicists have attempted to alter human gene pools by excluding people and groups judged to be inferior or ...
and
behavioral genetics
Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" c ...
when he was discussing the influence of
heredity
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic inform ...
and
environment on social advancement.
Galton was influenced by ''
On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' written by his
half-cousin, the evolutionary biologist
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
.
The view that humans acquire all or almost all their behavioral traits from "nurture" was termed ''
tabula rasa
''Tabula rasa'' (; "blank slate") is the theory that individuals are born without built-in mental content, and therefore all knowledge comes from experience or perception. Epistemological proponents of ''tabula rasa'' disagree with the doctri ...
'' ('blank tablet, slate') by
John Locke
John Locke (; 29 August 1632 – 28 October 1704) was an English philosopher and physician, widely regarded as one of the most influential of Age of Enlightenment, Enlightenment thinkers and commonly known as the "father of liberalism ...
in 1690. A ''blank slate view'' (sometimes termed ''blank-slatism'') in human
developmental psychology
Developmental psychology is the science, scientific study of how and why humans grow, change, and adapt across the course of their lives. Originally concerned with infants and children, the field has expanded to include adolescence, adult deve ...
, 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
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
, 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
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
loops have been found in which nature and nurture influence one another constantly, as seen in
self-domestication Self-domestication is the process of adaptation of for example wild animals to cohabiting with humans, without direct human selective breeding of the animals. The human self-domestication hypothesis argues that, like mammalian domesticates, humans h ...
. In
ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
and
behavioral genetics
Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" c ...
, researchers think nurture has an essential influence on nature. Similarly in other fields, the dividing line between an inherited and an acquired trait becomes unclear, as in
epigenetics
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
or
fetal development
Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...
.
History of debate
According to ''
Records of the Grand Historian
''Records of the Grand Historian'', also known by its Chinese name ''Shiji'', is a monumental history of China that is the first of China's 24 dynastic histories. The ''Records'' was written in the early 1st century by the ancient Chinese hist ...
'' (94 BC) by
Sima Qian
Sima Qian (; ; ) was a Chinese historian of the early Han dynasty (206AD220). He is considered the father of Chinese historiography for his ''Records of the Grand Historian'', a general history of China covering more than two thousand years b ...
, during
Chen Sheng Wu Guang uprising
The Chen Sheng and Wu Guang uprising (), July–December 209 B.C., was the first uprising against the Qin dynasty following the death of Qin Shi Huang. Led by Chen Sheng and Wu Guang, the uprising helped overthrow the Qin and paved the way for the ...
in 209 B.C.,
Chen Sheng
Chen Sheng (died January 208 BC), also known as Chen She ("She" being his courtesy name), posthumously known as Prince Yin, was the leader of the Dazexiang Uprising, the first rebellion against the Qin Dynasty. It occurred during the reign of ...
asked the rhetorical question as a call to war: "Are kings, generals, and ministers merely born into their kind?" ( zh, 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
John Locke (; 29 August 1632 – 28 October 1704) was an English philosopher and physician, widely regarded as one of the most influential of Age of Enlightenment, Enlightenment thinkers and commonly known as the "father of liberalism ...
's ''
An Essay Concerning Human Understanding
''An Essay Concerning Human Understanding'' is a work by John Locke concerning the foundation of human knowledge and understanding. It first appeared in 1689 (although dated 1690) with the printed title ''An Essay Concerning Humane Understan ...
'' (1690) is often cited as the foundational document of the ''
blank slate
''Tabula rasa'' (; "blank slate") is the theory that individuals are born without built-in mental content, and therefore all knowledge comes from experience or perception. Epistemological proponents of ''tabula rasa'' disagree with the doctri ...
'' view. In the ''Essay'', Locke specifically criticizes
René Descartes
René Descartes ( or ; ; Latinized: Renatus Cartesius; 31 March 1596 – 11 February 1650) was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Mathem ...
's claim of an
innate idea of
God
In monotheism, monotheistic thought, God is usually viewed as the supreme being, creator deity, creator, and principal object of Faith#Religious views, faith.Richard Swinburne, Swinburne, R.G. "God" in Ted Honderich, Honderich, Ted. (ed)''The Ox ...
that is universal to humanity. Locke's view was harshly criticized in his own time.
Anthony Ashley-Cooper, 3rd Earl of Shaftesbury
Anthony Ashley Cooper, 3rd Earl of Shaftesbury (26 February 1671 – 16 February 1713) was an English politician, philosopher, and writer.
Early life
He was born at Exeter House in London, the son of the future Anthony Ashley Cooper, 2nd E ...
, 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
Moral relativism or ethical relativism (often reformulated as relativist ethics or relativist morality) is used to describe several philosophical positions concerned with the differences in moral judgments across different peoples and cultures. ...
. By the 19th century, the predominant perspective was contrary to that of Locke's, tending to focus on "
instinct
Instinct is the inherent inclination of a living organism towards a particular complex behaviour, containing both innate (inborn) and learned elements. The simplest example of an instinctive behaviour is a fixed action pattern (FAP), in which a v ...
."
Leda Cosmides and
John Tooby noted that
William James
William James (January 11, 1842 – August 26, 1910) was an American philosopher, historian, and psychologist, and the first educator to offer a psychology course in the United States.
James is considered to be a leading thinker of the lat ...
(1842–1910) argued that humans have ''more''
instinct
Instinct is the inherent inclination of a living organism towards a particular complex behaviour, containing both innate (inborn) and learned elements. The simplest example of an instinctive behaviour is a fixed action pattern (FAP), in which a v ...
s 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
Free will is the capacity of agents to choose between different possible courses of action unimpeded.
Free will is closely linked to the concepts of moral responsibility, praise, culpability, sin, and other judgements which apply only to actio ...
in
moral philosophy
Ethics or moral philosophy is a branch of philosophy that "involves systematizing, defending, and recommending concepts of right and wrong behavior".''Internet Encyclopedia of Philosophy'' The field of ethics, along with aesthetics, concerns ma ...
. 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
John Leslie Mackie (25 August 1917 – 12 December 1981) was an Australian philosopher. He made significant contributions to the philosophy of religion, metaphysics, and the philosophy of language, and is perhaps best known for his views on m ...
) 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
Thomas Nagel (; born July 4, 1937) is an American philosopher. He is the University Professor of Philosophy and Law Emeritus at New York University, where he taught from 1980 to 2016. His main areas of philosophical interest are legal philosophy, ...
) 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 the environment, as a reaction to the strong focus on pure heredity in the wake of the triumphal success of Darwin's
theory of evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variatio ...
. During this time, the
social sciences
Social science is one of the branches of science, devoted to the study of societies and the relationships among individuals within those societies. The term was formerly used to refer to the field of sociology, the original "science of soci ...
developed as the project of studying the influence of culture in clean isolation from questions related to "biology.
Franz Boas
Franz Uri Boas (July 9, 1858 – December 21, 1942) was a German-American anthropologist and a pioneer of modern anthropology who has been called the "Father of American Anthropology". His work is associated with the movements known as historical ...
's ''
The Mind of Primitive Man
''The Mind of Primitive Man'' is a 1911 book by anthropologist Franz Boas which takes a critical look at the concept of primitive culture.Boas, Franz (1911). ''The Mind of Primitive Man''. The Macmillan Company The work challenged widely held racis ...
'' (1911) established a program that would dominate
American anthropology for the next 15 years. In this study, he established that in any given
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
,
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
,
language
Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of met ...
,
material
Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geologi ...
, and
symbolic culture, are
autonomous
In developmental psychology and moral, political, and bioethical philosophy, autonomy, from , ''autonomos'', from αὐτο- ''auto-'' "self" and νόμος ''nomos'', "law", hence when combined understood to mean "one who gives oneself one's ow ...
; 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
John Broadus Watson (January 9, 1878 – September 25, 1958) was an American psychologist who popularized the scientific theory of behaviorism, establishing it as a psychological school.Cohn, Aaron S. 2014.Watson, John B." Pp. 1429–1430 in ''T ...
in the 1920s and 1930s established the school of ''purist
behaviorism
Behaviorism is a systematic approach to understanding the behavior of humans and animals. It assumes that behavior is either a reflex evoked by the pairing of certain antecedent (behavioral psychology), antecedent stimuli in the environment, o ...
'' 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:During the 1940s to 1960s,
Ashley Montagu
Montague Francis Ashley-Montagu (June 28, 1905November 26, 1999) — born Israel Ehrenberg — was a British-American anthropologist who popularized the study of topics such as race and gender and their relation to politics and development. He ...
was a notable proponent of this purist form of behaviorism which allowed no contribution from heredity whatsoever: In 1951, Calvin Hall suggested that the dichotomy opposing nature to nurture is ultimately fruitless.
In ''
African Genesis
''African Genesis: A Personal Investigation into the Animal Origins and Nature of Man,'' usually referred to as ''African Genesis,'' is a 1961 nonfiction work by the American writer Robert Ardrey. It posited the hypothesis that man evolved on the A ...
'' (1961) and ''
The Territorial Imperative
''The Territorial Imperative: A Personal Inquiry Into the Animal Origins of Property and Nations'' is a 1966 nonfiction book by American writer Robert Ardrey. It describes the evolutionarily determined instinct among humans toward territoriality ...
'' (1966),
Robert Ardrey
Robert Ardrey (October 16, 1908 – January 14, 1980) was an American playwright, screenwriter and science writer perhaps best known for '' The Territorial Imperative'' (1966). After a Broadway and Hollywood career, he returned to his academi ...
argues for innate attributes of human nature, especially concerning
territoriality
In ethology, territory is the sociographical area that an animal consistently defends against conspecific competition (or, occasionally, against animals of other species) using agonistic behaviors or (less commonly) real physical aggression. ...
.
Desmond Morris
Desmond John Morris FLS ''hon. caus.'' (born 24 January 1928) is an English zoologist, ethologist and surrealist painter, as well as a popular author in human sociobiology. He is known for his 1967 book '' The Naked Ape'', and for his televis ...
in ''
The Naked Ape
''The Naked Ape: A Zoologist's Study of the Human Animal'' is a 1967 book by English Zoology, zoologist and ethology, ethologist Desmond Morris that looks at humans as a species and compares them to other animals. ''The Human Zoo (book), The Hu ...
'' (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
Edward Osborne Wilson (June 10, 1929 – December 26, 2021) was an American biologist, naturalist, entomologist and writer. According to David Attenborough, Wilson was the world's leading expert in his specialty of myrmecology, the study of an ...
(''
On Human Nature
''On Human Nature'' (1978; second edition 2004) is a book by the biologist E. O. Wilson, in which the author attempts to explain human nature and society through sociobiology. Wilson argues that evolution has left its traces on characteristics su ...
'', 1979).
The tool of
twin studies
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics a ...
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
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
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
Donald Olding Hebb (July 22, 1904 – August 20, 1985) was a Canadian psychologist who was influential in the area of neuropsychology, where he sought to understand how the function of neurons contributed to psychological processes such as learn ...
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
A cultural universal (also called an anthropological universal or human universal) is an element, pattern, trait, or institution that is common to all known human cultures worldwide. Taken together, the whole body of cultural universals is known ...
. 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
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
Richard Charles Lewontin (March 29, 1929 – July 4, 2021) was an American evolutionary biologist, mathematician, geneticist, and social commentator. A leader in developing the mathematical basis of population genetics and evolutionary theory, ...
,
Steven Rose and
Leon Kamin
Leon J. Kamin (December 29, 1927 – December 22, 2017) was an American psychologist known for his contributions to learning theory and his critique of estimates of the heritability of IQ. He studied under Richard Solomon at Harvard and contributed ...
criticise "
genetic determinism
Biological determinism, also known as genetic determinism, is the belief that human behaviour is directly controlled by an individual's genes or some component of their physiology, generally at the expense of the role of the environment, whether ...
" from a
Marxist
Marxism is a Left-wing politics, left-wing to Far-left politics, far-left method of socioeconomic analysis that uses a Materialism, materialist interpretation of historical development, better known as historical materialism, to understand S ...
framework, arguing that "Science is the ultimate legitimator of bourgeois ideology ... If
biological determinism
Biological determinism, also known as genetic determinism, is the belief that human behaviour is directly controlled by an individual's genes or some component of their physiology, generally at the expense of the role of the environment, whether i ...
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
Politics (from , ) is the set of activities that are associated with making decisions in groups, or other forms of power relations among individuals, such as the distribution of resources or status. The branch of social science that studies ...
or
ethically
Ethics or moral philosophy is a branch of philosophy that "involves systematizing, defending, and recommending concepts of right and wrong behavior".''Internet Encyclopedia of Philosophy'' The field of ethics, along with aesthetics, concerns ...
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
Judith Rich Harris (February 10, 1938 – December 29, 2018) was an American 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 pr ...
was heralded by
Steven Pinker
Steven Arthur Pinker (born September 18, 1954) is a Canadian-American cognitive psychologist, psycholinguist, popular science author, and public intellectual. He is an advocate of evolutionary psychology and the computational theory of mind.
P ...
as a book that "will come to be seen as a turning point in the
history of psychology
Psychology is defined as "the scientific study of behavior and mental processes". Philosophical interest in the human mind and behavior dates back to the ancient civilizations of Egypt, Persia, Greece, China, and India.
Psychology as a field of ...
." 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
Steven Arthur Pinker (born September 18, 1954) is a Canadian-American cognitive psychologist, psycholinguist, popular science author, and public intellectual. He is an advocate of evolutionary psychology and the computational theory of mind.
P ...
. 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
Dogma is a belief or set of beliefs that is accepted by the members of a group without being questioned or doubted. It may be in the form of an official system of principles or doctrines of a religion, such as Roman Catholicism, Judaism, Islam ...
linked to two other dogmas found in the dominant view of human nature in the 20th century:
# "
noble savage," in the sense that people are born good and corrupted by bad influence; and
# "
ghost in the machine
The "ghost in the machine" is a term originally used to describe and critique the notion of the mind existing alongside and separate to the body. In more recent times, the term has several uses, including the concept that the intellectual part of ...
," 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
![Sibling-correlation-422](https://upload.wikimedia.org/wikipedia/commons/0/09/Sibling-correlation-422.png)
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.
[ Pinker, Steven. 2002. '' The Blank Slate: The Modern Denial of Human Nature'' (1st ed.). ]Viking Press
Viking Press (formally Viking Penguin, also listed as Viking Books) is an American publishing company owned by Penguin Random House. It was founded in New York City on March 1, 1925, by Harold K. Guinzburg and George S. Oppenheim and then acquire ...
.
Lay summary
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
Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
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
Adoption is a process whereby a person assumes the parenting of another, usually a child, from that person's biological or legal parent or parents. Legal adoptions permanently transfer all rights and responsibilities, along with filiation, from ...
. In one kind of
adoption study
Adoption studies are one of the classic research methods of behavioral genetics and are an effective technique for assessing the interconnections of genetic and environmental variables in the elicitation of human qualities such as intelligence (IQ) ...
, 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:
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
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics ...
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
Human nature is a concept that denotes the fundamental dispositions and characteristics—including ways of thinking, feeling, and acting—that humans are said to have naturally. The term is often used to denote the essence of humankind, or ...
may arise from stochastic variations in prenatal development and is thus in no sense of the term "cultural".
Gene–environment interaction
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 (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino a ...
to partially suppress the genetic disease
phenylketonuria
Phenylketonuria (PKU) is an inborn error of metabolism that results in decreased metabolism of the amino acid phenylalanine. Untreated PKU can lead to intellectual disability, seizures, behavioral problems, and mental disorders. It may also re ...
. 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.
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
Mendelian inheritance (also known as Mendelism) is a type of biology, biological Heredity, inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, an ...
genetic disorders such as
Huntington's disease
Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...
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
A first language, native tongue, native language, mother tongue or L1 is the first language or dialect that a person has been exposed to from birth or within the critical period. In some countries, the term ''native language'' or ''mother tongu ...
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
Complex traits, also known as quantitative traits, are traits that do not behave according to simple Mendelian inheritance laws. More specifically, their inheritance cannot be explained by the genetic segregation of a single gene. Such traits show ...
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
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
, 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 Arthur Pinker (born September 18, 1954) is a Canadian-American cognitive psychologist, psycholinguist, popular science author, and public intellectual. He is an advocate of evolutionary psychology and the computational theory of mind.
P ...
likewise described several examples:When traits are determined by a complex interaction of
genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
and environment it is possible to measure the
heritability
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
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 are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two ...
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 are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
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
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. Th ...
.
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
Attachment theory is a psychological, evolutionary and ethological theory concerning relationships between humans. The most important tenet is that young children need to develop a relationship with at least one primary caregiver for normal ...
. 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
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
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
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...
.
Heritability of intelligence
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
Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" co ...
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
The American Psychological Association (APA) is the largest scientific and professional organization of psychologists in the United States, with over 133,000 members, including scientists, educators, clinicians, consultants, and students. It ha ...
'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.[Plomin, R., J. C. DeFries, G. E. McClearn, and P. McGuffn. 2008. ''Behavioral Genetics'' (5th ed.). New York: Worth Publishers. ]
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.
Personality traits
Personality
Personality is the characteristic sets of behaviors, cognitions, and emotional patterns that are formed from biological and environmental factors, and which change over time. While there is no generally agreed-upon definition of personality, mos ...
is a frequently cited example of a heritable trait that has been studied in twins and adoptees using behavioral genetic
Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" co ...
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. 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.
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
Genomics
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 genomic sequencing
DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. The ...
, 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. An example of a visible human trait for which the precise genetic basis of differences are relatively well known is eye color
Eye color is a polygenic phenotypic character determined by two distinct factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.
In humans, the pi ...
.
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
David Reimer (born Bruce Peter Reimer; 22 August 1965 – 4 May 2004) was a Canadian man born male but raised as a girl following medical advice and intervention after his penis was severely injured during a botched circumcision in infancy. ...
), genomics has provided solid evidence that both sex and gender identities are primarily influenced by genes:
Linkage and association studies
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.
See also
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Meta-analysis of twin correlations and heritability
{{Authority control
20th-century controversies
Human behavior
Human development
Personality theories
Race and intelligence controversy
Biology controversies
Dilemmas
Human genetics
Psychology controversies
Evolutionary psychology