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Nature versus nurture

Nature versus nurture is a popular phrase used to describe debates over the relative degrees to which one's genetic makeup ("nature") and one's life experiences ("nurture") influence one's traits or attributes. A wide variety of traits have been considered in such debates, including personality, sexual orientation, political orientation, intelligence, and propensity for violence or criminality.

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. Additionally, although childhood experience (especially early childhood experience) is often regarded as more influential in who one becomes than post-childhood experience, a liberal interpretation of "nurture" might count all life experience as "nurture".

Uncomplicated cases

In a few clear-cut cases, it makes sense to say that a trait is due almost entirely to nature, or almost entirely to nurture. In the case of highly penetrant genetic diseases, such as Huntington's disease, nature seems to be the right answer; basically, you will get the disease if and only if you have the corresponding disease-causing allele (gene variant). In the case of which particular language you speak, nurture seems to be the right answer; linguists have found that any "normal" child can learn any human language. With most interesting traits, however, there is probably an intermediate mix of nature and nurture, and people may disagree wildly about the relative importance of each.

How to compare the effects of nature and nurture, and why this is hard

Current thinking in biology discredits the notion that genes alone can determine a trait because genes are never sufficient in isolation. Rather, 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 to psychological traits, such as intelligence and personality; yet these traits may be largely influenced by environment in other circumstances.

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. Often this reduces to calculating the heritability of a trait.

In many cases the difficulty of creating situations suitable for testing environmental and genetic influence on traits has been compensated for by finding existing populations that reflect the experimental setting the researcher wishes to create. One way to do this is the study of twins. Some studies compare identical twins to fraternal twins, while others study identical twins reared apart.

For example, many twin studies have made use of identical twins (who have the same genetic makeup) who were raised in differing environments in order to control for genetic effects: that is, any variation between twins is clearly attributable to the environment, allowing the researcher to quantify the effects of the environment by measuring variance of a trait between twins. Identical twins raised separately may have experienced quite different environments; yet many studies have often been found that they live similar lives, have similar personalities and similar levels of intelligence.

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. Even using experiments like those described above, it can be very difficult to determine convincingly the relative contribution of genes and environment.

Twin studies have highlighted another complication to the nature versus nurture debate. The effects of nurture can be further divided into shared and non-shared. Shared environmental factors are those experienced by siblings raised together. Non-shared environmental factors are not shared by siblings (i.e. unique experiences). In many cases non-shared environmental effects have been found to out-weight 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.

Moral difficulties: eugenics, etc..

Modern science, however, tends to frown upon giving 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 validity has often been assumed as a prerequisite in various incarnations of the nature versus nurture debate. Genetics, long having been used as "scientific" justification for genocide, or race-based discrimination.

...Steven Pinker: moral ideals directing science?

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 whether our traits are determined by genes or environment has little impact on the our free will. That is, and environmental determinant can be just as inescapable as a genetic determinant. If "nature" and "nurture" together have so much influence on who we are, then is there such a thing as "free will"?

Myths and mysteries

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.

Misc

The concurrent development phenomenon: why do identical twins, separated at birth, grow to look and act so similarly?

A number of social issues exist, especially in education and in law with regards to culpability.

See also