HUMAN NATURE : SOME APPROACHES

The 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 family upbringing and socioeconomic status, 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]

Philosophical Approach

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, Gods, 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 with new understandings 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.

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 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.
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.

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.