Читать книгу Lifespan Development - Tara L. Kuther - Страница 135

Behavioral geneticists devise ways of estimating the heritability of specific traits and behaviors. Heritability refers to the extent to which variation among people on a given characteristic is due to genetic differences. The remaining variation not due to genetic differences is instead a result of the environment and experiences. Heritability research therefore examines the contributions of the genotype but also provides information on the role of experience in determining phenotypes (Plomin et al., 2016). Behavioral geneticists assess the hereditary contributions to behavior by conducting selective breeding and family studies.

Using selective breeding studies, behavioral geneticists deliberately modify the genetic makeup of animals to examine the influence of heredity on attributes and behavior. For example, mice can be bred to very physically active or sedentary by mating highly active mice only with other highly active mice and, similarly, breeding mice with very low levels of activity with each other. Over subsequent generations, mice bred for high levels of activity become many times more active than those bred for low levels of activity (Knopik, Neiderhiser, DeFries, & Plomin, 2017). Selective breeding in rats, mice, and other animals such as chickens has revealed genetic contributions to many traits and characteristics, such as aggressiveness, emotionality, sex drive, and even maze learning (Plomin et al., 2016).

Behavioral geneticists conduct family studies to compare people who live together and share varying degrees of relatedness. Two kinds of family studies are common: twin studies and adoption studies (Koenen, Amstadter, & Nugent, 2012). Twin studies compare identical and fraternal twins to estimate how much of a trait or behavior is attributable to genes. If genes affect the attribute, identical twins should be more similar than fraternal twins because identical twins share 100% of their genes, whereas fraternal twins share about only 50%. Adoption studies, on the other hand, compare the degree of similarity between adopted children and their biological parents whose genes they share (50%) and their adoptive parents with whom they share no genes. If the adopted children share similarities with their biological parents, even though they were not raised by them, it suggests that the similarities are genetic.

Adoption studies also shed light on the extent to which attributes and behaviors are influenced by the environment. For example, the degree to which two genetically unrelated adopted children reared together are similar speaks to the role of environment. Comparisons of identical twins reared in the same home with those reared in different environments can also illustrate environmental contributions to phenotypes. If identical twins reared together are more similar than those reared apart, an environmental influence can be inferred.