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Nature versus nurture is a shorthand expression for debates about the relative importance of an individual's innate qualities ("nature") versus personal experiences ("nurture") in determining or causing individual differences in physical and behavioral traits. For a discussion of nature versus nurture in human universals, see also psychological nativism.

History

While classical theories regarding these matters were primarily concerned with the line between that which was voluntary (the ego, the self, and the personal will) and the involuntary (of Nature, God, etc.), this view was self-centric, which is to say deferential to authorities over the personal concepts; i.e. religious teaching and doctrine.

As science developed an understanding of life's elemental nature (determined in its nature and behavior by the behaviors of constituent elements and prevailing physical phenomena, e.g. atoms, molecules, genes, force, and time), the categories that classical formalism defined came to be seen as arbitrary, and the trend of science since has been to develop away from the human-centered view to a more elemental, deterministic, reductionist view.

Scientific culture to this day functions within a social boundary imposed by the prevalence among laypeople of classical views. This boundary contains the impact of any scientific discoveries or observations on matters of human society. Thus nature versus nurture debates can be seen as attempts to fit new scientific ideas and developments into the classical formalist and self-based mold, since these debates arose from problems associated with reconciling the formalist notions of classical theories with emerging theories and new data. The gap between prevailing scientific opinion and prevailing lay opinion is reflected in popular science.

Advocates of a formalist view may discount completely the influence of one contributor or the other for the sake of some agenda, for example denying the influence of "nature" in order to preserve the idea of free will as the sole important determinant of behavior, a notion held to be of central importance in many religious, ethical, and legal systems, particularly in establishing culpability.

Definitions of nature and nurture

Although "nurture" may have historically referred mainly to the care given to children by their parents, any environmental (not genetic) factor also would count as "nurture" in a contemporary nature versus nurture debate, including one's childhood friends, one's early experiences with television, and one's experience in the womb. Indeed, a substantial source of environmental input to human nature may arise from stochastic variations in prenatal development.

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. 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. These interactional traits are, in a sense, "complexly determined".

Examples of environmental, interactional, and genetic traits are:

Predominantly Environmental Interactional Predominantly Genetic
Language Height Blood type
Religion Weight Eye color
etc. Skin color etc.
Sexual orientation
IQ test results
GxE-herit-fig2

The "two buckets" view of heritability.

GxE-herit-fig1

More correct "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.

The difficulties in estimating heritability

Sibling-correlation-422

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 heritibility, 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, meaning roughly that neither "nature" nor "nurture" determines everything about an individual.

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. 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).

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. Typically, two kinds of environmental effects are distinguished: shared family effects (those shared by siblings, making them more similar) and nonshared effects (which uniquely affect individuals, making siblings different). Although they are genetically identical and share the same family environment, identical twins reared together do not have identical personalities. These differences are caused entirely (by definition) by nonshared environmental effects. Adoption studies also directly measure the strength of shared family effects. Adopted siblings share only family environment. Unexpectedly, adoption studies indicate that by adulthood the personalities of adopted siblings are no more similar than random pairs of strangers. This means 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) 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".

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

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, 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 which 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.

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.

Philosophical difficulties: 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; however, many are accused of doing so.

Others have pointed out that the premise of the "nature versus nurture" debate seems to negate the significance of free will. 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.

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 preposterous, 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

References

  • 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.
  • Harrison PJ, Owen MJ. (2003) Genes for schizophrenia? Recent findings and their pathophysiological implications. Lancet, 361(9355), 417–9.
  • 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 0716751593.


  • Ridley, M. (2003). Nature Via Nurture : Genes, Experience, and What Makes Us Human, HarperCollins. ISBN 0060006781.
(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.
  • Burks, B.S. (1928) The relative influence of nature and nurture upon mental development: a comparative study of foster-parent-foster-child resemblance and true-parent-true-child resemblance. In: 27th Yearbook of the National Society for the Study of Education, Bloomington, Ill: Public School Publishing Co.
The development of phenotype
Key concepts: Genotype-phenotype distinction | Norms of reaction | Gene-environment interaction | Heritability | Quantitative genetics
Genetic architecture: Dominance relationship | Epistasis | Polygenic inheritance | Pleiotropy | Plasticity | Canalisation | Fitness landscape
Non-genetic influences: Epigenetic inheritance | Epigenetics | Maternal effect | dual inheritance theory
Developmental architecture: Segmentation | Modularity
Evolution of genetic systems: Evolvability | Mutational robustness | Evolution of sex
Influential figures: C. H. Waddington | Richard Lewontin
Debates: Nature versus nurture
List of evolutionary biology topics
da:Arv og miljø
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