Читать книгу Abnormal Psychology - William J. Ray - Страница 137

One type of genetics research involving psychopathology is behavioral genetics, which is the study of genetic and environmental contributions to organisms’ behavior (see G. Carey, 2003; DiLalla, 2004; Kendler, Jaffee, & Romer, 2011; Plomin, DeFries, Knopik, & Neiderhiser, 2012, for overviews). One large question is the manner in which genes and the environment work together to shape behavior. Researchers use a variety of behavioral genetic approaches to quantify the amount of variance, which can be attributed to genetic and environmental influences.

behavioral genetics: the study of genetic and environmental contributions to organisms’ behavior

gene by environment interaction: the possibility that individuals with different genotypes may respond to the same environment in different ways

One traditional distinction has been between gene by environment interaction as opposed to gene by environment correlations (Plomin, DeFries, & Loehlin, 1977). Gene by environment interactions refer to the possibility that individuals with different genotypes may respond to the same environment in different ways. For example, some children respond to stress differently from others based on their genotype. As noted, mistreatment as a child influenced some boys differently from others later in adulthood (Caspi et al., 2002). Those boys who were mistreated in childhood and had a particular form of the MAOA gene were more likely to be violent and engage in a variety of antisocial behaviors as adults.

A gene by environment correlation concerns how certain genotypes and certain environments occur together. For example, sensation seeking as a personality trait has been shown to be inherited. Those individuals who are sensation seekers are more likely to put themselves in high-risk environments such as mountain climbing or auto racing. In this example, it is more difficult to determine the amount of variance attributed to genetic influences and environmental influences as separate factors. Thus, it is possible for genetic and environmental factors to influence each other in subtle ways.

gene by environment correlations: how certain genotypes and certain environments occur together

twin studies: a major paradigm of behavioral genetics involving examination and understanding of critical factors related to genetic influences by studying twins

monozygotic (MZ) twins: identical twins resulting from the zygote (fertilized egg) dividing during the first 2 weeks of gestation

dizygotic (DZ) twins: twins who arise from the situation in which two different eggs are fertilized by two different spermatozoa; these are called fraternal twins since their shared genes are approximately 50%—the same as that between any two siblings

adoption study: research into the phenomenon where dizygotic (DZ) and monozygotic (MZ) twins have been raised apart, providing insights into the environmental and genetic influences on human development and behavior

One major paradigm of behavioral genetics involves twin studies. Twins offer an occurrence in nature that allows for understanding critical factors related to genetic influences. This is largely based on the fact that there are two types of twins. Monozygotic (MZ) twins are identical twins resulting from the zygote (fertilized egg) dividing during the first 2 weeks of gestation. Because they both come from the same egg, their genes are identical. Dizygotic (DZ) twins, on the other hand, arise from the situation in which two different eggs are fertilized by two different spermatozoa. These are called fraternal twins, since their shared genes are approximately 50%—the same as that between any two siblings. DZ twins can be either same sex or opposite sex, whereas MZ twins must always be the same sex. By comparing the psychological traits of MZ and DZ twins, it is possible to obtain an estimate of heritability.

A classic research design is to compare the responses of MZ twins with DZ twins on particular behavioral traits such as intelligence or personality characteristics. Since it is assumed that both DZ and MZ twins would have had similar environmental influences in their family, any differences between MZ and DZ twins would be seen to be the result of genetic influences. For example, Gottesman (1991) has studied schizophrenia with this design. In these studies, a particular MZ twin was more likely (.50) to have schizophrenia if the other twin also did. In DZ twins, this was not the case.

Statistically, researchers examine the degree to which twins are identical to each other as a function of genetic influences and environmental influences. To answer this question, researchers create correlation coefficients for MZ twins and DZ twins. This correlation reflects how similar each type of twin is on a particular trait. From this, it is possible to determine the percentage of contribution to the trait that comes from environmental influences and the percentage of contribution that comes from genetic influences. For example, personality factors such as extraversion have been shown to have a 50% contribution of genetic factors and a 50% contribution of environmental factors.

Studies of identical twins have shown they share similarities in IQ, temperament, and leisure time interests, whether they are raised together or apart.

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Another important type of behavioral genetics research is the adoption study. This is the situation where DZ and MZ twins have been raised apart. In the United States since 1979, a series of twins who were separated in infancy and reared apart have been studied by researchers at the University of Minnesota (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990).

In work with identical twins, researchers studying over 100 pairs of twins found that about 70% of the variance in IQ could be associated with genetic factors (Bouchard et al., 1990). Later studies have supported this finding. However, if the child’s family lived in poverty, the degree of association dropped drastically. Although it is not surprising to find IQ or temperament to have genetic associations, it was intriguing to see that the leisure time interests (e.g., sporting activities, hobbies, reading preferences, etc.) of each twin in the pair were similar whether the twins were reared together or reared apart.

linkage analysis: an examination of generations of families that looks for the association between certain deoxyribonucleic acid (DNA) marker alleles and particular traits

A third type of study is referred to as a linkage analysis. Linkage analysis examines generations of families and looks for the association between particular deoxyribonucleic acid (DNA) marker alleles and particular traits. This is commonly done for disorders that run in families. This approach is often used for studying psychological and physiological disorders such as depression or bipolar disorder. As part of this type of research, scientists may examine the manner in which traits associated with the disorder are also apparent in first- and second-degree relatives.