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The nature versus nurture debates concern the relative importance of an individual's innate qualities ("nature", i.e. nativism, or philosophical empiricism, innatism) versus personal experiences ("nurture") in determining or causing individual differences in physical and behavioral traits. The view that humans acquire all or almost all their behavioral traits from "nurture" is known as tabula rasa ("blank slate"). 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to play such interacting roles in development, many modern psychologists consider the question naive - representing an outdated state of knowledge^[1][2][3]. For a discussion of nature versus nurture in language and other human universals, see also psychological nativism.

Scientific approach

In order to disentangle the effects of genes and environment, behavioral geneticists perform adoption and twin studies. Behavioral geneticists do not generally use the term "nurture" in order to explain that portion of the variance for a given trait (such as IQ or the Big Five personality traits) that can be attributed to environmental effects. Instead, two different types of environmental effects are distinguished: 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). In order to express the portion of the variance that is due to the "nature" component, behavioral geneticists generally refer to the heritability of a trait.

With regard to the Big Five personality traits 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. ^[4] 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-nominated book The Nurture Assumption, author Judith Harris 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 ^{[5] [6]}

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 been expanded in order to include the influences on development arising from prenatal, parental, extended family and peer experiences, extending to influences such as media, marketing and socio-economic status. Indeed, a substantial source of environmental input to human nature may arise from stochastic variations in prenatal development.^{[citation needed]}

Heritability estimates

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

Current thinking in biology discredits the notion that genes alone can determine a trait because genes are never sufficient in isolation. At the molecular level, DNA interacts in complex ways with signals from other genes and from the environment. At the level of individuals, particular genes influence the development of a trait in the context of a particular environment. Thus, measurements of the degree to which a trait is influenced by genes versus environment will depend on the particular environment and genes examined. In many cases, it has been found that genes may have a substantial contribution, including psychological traits such as intelligence and personality^[7]. Yet, these traits may be largely influenced by environment in other circumstances, such as environmental deprivation.

A researcher seeking to quantify the influence of genes or environment on a trait needs to be able to separate the effects of one factor away from that of another. This kind of research often begins with attempts to calculate the heritability of a trait. Heritability quantifies the extent to which variation among individuals in a trait is due to variation in the genes those individuals carry. In animals where breeding and environments can be controlled experimentally, heritability can be determined relatively easily. Such experiments would be unethical for human research. This problem can be overcome by finding existing populations of humans that reflect the experimental setting the researcher wishes to create.

One way to determine the contribution of genes and environment to a trait is to study twins. In one kind of study, identical twins reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. In another kind of twin study, identical twins reared together (who share family environment and genes) are compared to fraternal twins reared together (who also share family environment but only share half their genes). Another condition that permits the disassociation of genes and environment is adoption. In one kind of adoption study, biological siblings reared together (who share the same family environment and half their genes) are compared to adoptive siblings (who share their family environment but none of their genes).

Some have rightly pointed out that environmental inputs affect the expression of genes. This is one explanation of how environment can influence the extent to which a genetic disposition will actually manifest. The interactions of genes with environment, called gene-environment interaction, 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.

Interaction of genes and environment

In only a very few cases is it fair to say that a trait is due almost entirely to nature, or almost entirely to nurture.^{[citation needed]} In the case of most diseases now strictly identified as genetic, such as Huntington's disease, there is a better than 99.9% correlation between having the identified gene and the disease and a similar correlation for not having either. On the other hand, such traits as one's native language are entirely 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 psychological traits however, there is an intermediate mix of nature and nurture, and opinions about the relative importance of each will often vary widely.

Examples of environmental, interactional, and genetic traits are:

The "two buckets" view of heritability.

More realistic "homogenous mudpie" view of heritability.

Steven Pinker (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 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.

Nature versus nurture in the IQ debate

Evidence suggests that family environmental factors may have an effect upon childhood IQ, 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.^[8] 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 (0.86), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (~0.0). ^[9] Consequently, in the context of the "nature versus nurture" debate, the "nature" component appears to be much more important than the "nurture" component in explaining IQ variance in the general adult population of the United States.

Nature versus nurture in personality traits

Personality is a frequently cited example of a heritable trait that has been studied in twins and adoptions. 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. 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. Unexpectedly, some adoption studies indicate that by adulthood the personalities of adopted siblings are no more similar than random pairs of strangers. This would mean that shared family effects on personality are zero by adulthood. As is the case with personality, 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"^{[citation needed]}.

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 heritability measures of 40 to 50 percent. Subsequent developmental genetic analyses have found that genetic contribution to intelligence increases over a lifespan,^[10][11][12] reaching a heritability of 80 percent 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 highly heritable traits, it is now possible to search for individual genes that contribute to variation in that trait. For example, several research groups have identified genetic loci that contribute to schizophrenia (Harrison and Owen, 2003).

Moral difficulties: eugenics, etc.

Some observers believe that modern science tends to give too much weight to the nature side of the argument, in part because of social consciousness. Historically, much of this debate has had undertones of racist and eugenicist policies — the notion of race as a scientific truth has often been assumed as a prerequisite in various incarnations of the nature versus nurture debate. In the past, heredity was often used as "scientific" justification for various forms of discrimination and oppression along racial and class lines. Works published in the United States since the 1960s that argue for the primacy of "nature" over "nurture" in determining certain characteristics, such as The Bell Curve, have been greeted with considerable controversy and scorn.

A critique of moral arguments against the nature side of the argument could be that they cross the is-ought gap. That is, they apply values to facts. However, such appliance appears to construct reality. Belief in biologically determined stereotypes and abilities has been shown to increase the kind of behavior that is associated with such stereotypes and to impair intellectual performance through, among other things, the stereotype threat phenomenon.

Philosophical difficulties

Are the traits real?

It is sometimes a question whether the "trait" being measured is even a real thing. Much energy has been devoted to calculating the heritability of intelligence (usually the I.Q., or intelligence quotient), but there is still some disagreement as to what exactly "intelligence" is.

Biological determinism

If genes do contribute substantially to the development of personal characteristics such as intelligence and personality, then many wonder if this implies that genes determine who we are. Biological determinism is the thesis that genes determine who we are. Few if any scientists would make such a claim^{[citation needed]}; however, many are accused of doing so. Defining the words "who" and "am" also present philosophical difficulties.

Others have pointed out that the premise of the "nature versus nurture" debate seems to negate the significance of free will^{[citation needed]}. More specifically, if all our traits are determined by our genes, by our environment, by chance, or by some combination of these acting together, then there seems to be little room for free will. In any case, this line of reasoning suggests that the "nature versus nurture" debate tends to exaggerate the degree to which individual human behavior can be predicted based on knowledge of genetics and the environment. It should also be pointed out that biology may determine our abilities, but free will still determines what we do with our abilities.

Is the problem real?

Many scientists feel that the very question opposing nature to nurture is a fallacy. Already in 1951, Calvin Hall in his seminal chapter^[13] remarked that the discussion opposing nature and nurture was fruitless. If an environment is changed fundamentally, then the heritability of a character changes, too. Conversely, if the genetic composition of a population changes, then heritability will also change. As an example, we may use phenylketonuria (PKU), which causes brain damage and progressive mental retardation. PKU can be treated by the elimination of phenylalanine from the diet. Hence, a character (PKU) that used to have a virtually perfect heritability is not heritable any more if modern medicine is available. Similarly, within, say, an inbred strain of mice, no genetic variation is present and every character will have a zero heritability. If the complications of gene-environment interactions and correlations (see above) are added, then it appears to many that heritability, the epitome of the nature-nurture opposition, is "a station passed".^[14]

Myths about identity

Within the debates surrounding cloning, for example, is the far-fetched contention that a Jesus or a Hitler could be "re-created" through genetic cloning. Current thinking finds this largely inaccurate, and discounts the possibility that the clone of anyone would grow up to be the same individual due to environmental variation. For example, like clones, identical twins are genetically identical, and unlike the hypothetical clones share the same family environment, yet they are not identical in personality and other traits.

See also

Look up nurture in
Wiktionary, the free dictionary.

• Cultural anthropology
• Diathesis-stress model
• Eugenics
• Genetic determinism
• Genetics and violence
• Human nature
• Onion Skin Model
• Inheritance of intelligence
• Language acquisition
• Nurture Assumption, The (book)
• Race and crime
• Reimer, David
• Social determinism
• Stereotype threat
• Tabula rasa
• Twin study
• Judith Harris
• Race and genetics

References

1. ^ Ridley, M. (2003) Nature Via Nurture: Genes, Experience, and What Makes us Human. Harper Collins. ISBN 0-00-200663-4
2. ^ Carlson, N. R. et al. (2005) Psychology: the science of behaviour (3rd Canadian ed) Pearson Ed. ISBN 0-205-45769-X
3. ^ Westen, D. (2002) Psychology: Brain, Behavior & Culture. Wiley & Sons. ISBN 0-471-38754-1
4. ^ DeFries, J. C., McGuffin, P., McClearn, G. E., Plomin, R. (2000) Behavioral Genetics 4th ED. W H Freeman & Co.
5. ^ http://72.14.253.104/search?q=cache:etWy56SPQQYJ:home.att.net/~xchar/tna. Website for "The Nurture Assumption."
6. ^ http://home.att.net/~xchar/tna/edge2006.htm
7. ^ Plomin, R., DeFries, J. C., McClearn, G. E., & McGuffin, P. 2001. Behavioral Genetics. (4th Edition). New York: Worth Publishers.
8. ^ * Plomin, R., DeFries, J. C., McClearn, G. E. and McGuffin, P. (2001). Behavioral Genetics (4th Ed.). New York: Freeman. ISBN 0-7167-5159-3. 
9. ^ Bouchard TJ Jr. Genetic and environmental influences on adult intelligence and special mental abilities. Hum Biol. 1998 Apr;70(2):257-79
10. ^ Plomin, R. 2004. Intelligence: genetics, genes, and genomics. Journal of Personality and Social Psychology 86 112-129
11. ^ M. McGue, T. J. Bouchard Jr., W. G. Iacono, & D. T. Lykken (1993) Behavioral Genetics of Cognitive Ability: A Life-Span Perspective, in Nature, Nurture, and Psychology, by R. Plomin & G. E. McClearn (Eds.) Washington, DC: American Psychological Association
12. ^ R. Plomin, D. W. Fulker, R. Corley, & J. C. DeFries (1997) Nature, Nurture and Cognitive Development from 1 to 16 years: A Parent-Offspring Adoption Study Psychological Science 8 442–447
13. ^ C. S. Hall (1951) The Genetics of Behavior, in Handbook of Experimental Psychology, by S. S. Stevens (Ed.) New York, NY, USA: John Wiley and Sons, pp. 304-329
14. ^ W. E. Crusio (1990) Estimating heritabilities in quantitative behavior genetics: A station passed. Behavioral and Brain Sciences 13 127-128

• Alarcon, M., Plomin, R., Fulker, D.W., Corley, R. & DeFries, J.C. (1998). Multivariate path analysis of specific cognitive abilities: data at 12 years of age in the Colorado Adoption Project. Behavior Genetics 28:255-264.
• Jang, K.L., McCrae, R.R., Angleitner, A. Riemann, R. & Livesley, W.J. (1998). Heritability of facet-level traits in a cross-cultural twin sample: support for a hierarchical model of personality. Journal of Personality and Social Psychology 74:1556-1565.
• Joseph, J. (2004)The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope.New York: Algora. (2003 United Kingdom Edition by PCCS Books)
• Joseph, J. (2006). The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes.New York: Algora.
• Harrison PJ, Owen MJ. (2003) Genes for schizophrenia? Recent findings and their pathophysiological implications. Lancet, 361(9355), 417–9.
• Neill, J. T. (2004). Nature vs nurture in intelligence. Wilderdom.
• Pinker, S. (2004) Why nature & nurture won't go away. Dædalus.
• Plomin, R., Fulker, D. W., Corley, R. & DeFries, J. C. (1997). Nature, nurture and cognitive development from 1 to 16 years: a parent-offspring adoption study. Psychological Science 8:442-447.
• Plomin, R., DeFries, J. C., McClearn, G. E. and McGuffin, P. (2001). Behavioral Genetics (4th Ed.). New York: Freeman. ISBN 0-7167-5159-3. 
• Ridley, M. (2003). Nature Via Nurture : Genes, Experience, and What Makes Us Human. HarperCollins. ISBN 0-06-000678-1.  (republished under the title The Agile Gene : How Nature Turns on Nurture)
• Wahlsten, D. (1997). Leilani Muir versus the Philosopher King: eugenics on trial in Alberta. Genetica 99: 185-198.
• At least two Science Fiction novels have plots which bear on this debate (in very different ways from each other): Cyteen by C. J. Cherryh (1988) and The Coming of the Quantum Cats by Frederik Pohl (1986).