Nature Versus Nurture - Scientific Approach

Scientific Approach

"Many properties of the brain are genetically organized, and don't depend on information coming in from the senses.” - Steven Pinker

To disentangle the effects of genes and environment, behavioral geneticists perform adoption and twin studies. These seek to decompose the variance in a population into genetic and environmental components. This move from individuals to populations makes a critical difference in the way we think about nature and nurture. This difference is perhaps highlighted in the quote attributed to psychologist Donald Hebb who is said to have once answered a journalist's question of "which, nature or nurture, contributes more to personality?" by asking in response, "Which contributes more to the area of a rectangle, its length or its width?" For a particular rectangle, its area is indeed the product of its length and width. Moving to a population, however, this analogy masks the fact that there are many individuals, and that it is meaningful to talk about their differences.

Scientific approaches also seek to break down variance beyond these two categories of nature and nurture. Thus rather than "nurture", behavior geneticists distinguish shared family factors (i.e., those shared by siblings, making them more similar) and nonshared factors (i.e., those that uniquely affect individuals, making siblings different). To express the portion of the variance due to the "nature" component, behavioral geneticists generally refer to the heritability of a trait.

With regard to personality traits and 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.

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.

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