One of the goals of the social sciences is to explain human behavior. Nature, which may be defined in terms of inheritance, biological background, and genetic makeup, must be taken into account, along with nurture, which may be defined in terms of experience and learning.
Historically, nature and nurture represented a dichotomy. The ancient Greeks, for example, debated the role of nature and nurture as influences on character and human nature. Since that time, nativists have argued that human personality, intelligence, and capabilities are tied to a person’s biological background. Empiricists have countered with arguments that each person is a tabula rasa, or a blank slate, and experience determines who humans are and what they can do. Learning theories that developed during the 1950s through the 1970s were sometimes radically empiricist; some theories suggested that any individual is capable of just about anything, if the experience is optimal.
In the first half of the twentieth century, the nature and nurture debate focused on personality and intelligence. Scientific methods were brought to bear on the topic, and there was a shift away from "nature or nurture" to a discussion over the role played by each. One of the key concepts resulting from this work, known as a range of reaction, captures the possible interaction between nature and nurture. In particular, a range of possibilities is genetically determined, and environment and experience determine how much of the potential is fulfilled. For example, certain individuals might have the genetic potential to be anywhere from 5 feet 6 inches to 5 feet 11 inches tall, but their experience (for example, exercise, nutrition) determines the height within that range to which they eventually grow.
A genotype is defined as the genetic constitution (and potential) of an individual. A phenotype, in contrast, is the observable characteristics resulting from the interaction of nature and nurture. Nature and nurture may interact such that experience operates within the potentials set by the genotype, as implied by the range of reaction. But nature and nurture may also interact such that genetic potentials influence the environment. In simple terms, a child may inherit a particular talent, but the talent is immature; it is mere potential. Parents, teachers, or coaches may recognize the potential and do what they can to support it. In this case, the genetic potential actually determines which experiences will be provided. Nature and nurture interact in a bidirectional fashion.
The interplay of nature and nurture has been studied extensively in behavioral genetics. Twins are often studied by behavioral geneticists. Monozygotic twins are genetically identical; they developed in utero from one fertilized egg cell. Fraternal or dizygotic twins, in contrast, developed from two egg cells and are consequently only 50 percent similar genetically. Knowing the precise genetic contributions allows researchers to make inferences about the impact of nature and nurture. In the clearest case, monozygotic twins who have been reared apart (for instance, separated before the age of six months) are, later in life, compared in terms of IQ or various personality traits. If genes play a significant role, the twins should be very similar, even though they had different experiences. The magnitude of the genetic contribution is calculated with a correlation, known as the heritability index. Empirical results suggest that monozygotic twins have the highest heritability, sometimes as high as .70, even when reared apart. Other siblings, including dizygotic twins, have much lower heritability indices (usually below .50).
Similar support for a genetic contribution to IQ comes from adoption studies. The logic parallels that of twin studies, only here the comparisons are between biological parents and their children (who share 50 percent of their genetic makeup) and foster or adoptive parents (who are dissimilar genetically to their adopted children but share the same environment with them). As with twin studies, the genetic contribution is quite apparent in adoption studies. Biological parents tend more often to have IQs that are similar to their children than do foster or adoptive parents.
The most ambitious investigation into the nature/nurture debate is the U.S. Human Genome Project. The project started in 1990 with the general objective of identifying all the genes in human DNA. There may be 25,000 such genes, with literally billions of chemical sequences. In addition to investigations of human DNA, the project examined mice, fruit flies, and even bacteria. The project was considered to have been completed in 2003 although research of the data is ongoing.
There are numerous political ramifications to the study of nature and nurture. Francis Galton (18221911), a cousin of Charles Darwin (1809-1882), believed strongly in the inheritance of ability and promoted the advantages of eugenics, or the strengthening of desirable characteristics using selective mating. Although eugenics is contrary to an appreciation of diversity and equality, various factions nonetheless support it. There is, for example, one well-known sperm bank in California that only deals with highly able donors and recipients. The implications of genetic research for medicine and clinical treatments are likely to have more practical importance.
In most cases, traits and capabilities are polygenetic; that is, more than one gene is responsible for them. Great headway has been made with the identification and even engineering of various genes and gene combinations. Modern genetics is increasingly precise about the contributions of nature and the biological influences on human behavior, and the inferences of behavioral genetics are no longer necessary. Alcoholism, autism, creative potential, and many other human tendencies have been examined and the influence of specific genes determined. Addiction, for example, seems to be strongly tied to the presence of a gene allele called D2R2, which is responsible for dopamine reception. Only about 20 percent of the population has that allele, and thus a proclivity toward addiction. Clearly, some of the most important implications of research on genetics are those within medicine.