LINKAGES Psychological Research and Behavioral Genetics
37 Research Methods in Psychology LINKAGES Is behavior inﬂuenced by our genes or by our environment? (a link to Biology and Behavior) behavioral genetics The study of how genes and environments combine to affect behavior and mental processes. O LINKAGES ne of the most fascinating and difﬁcult challenges in psychology is to Psychological Research and ﬁnd research methods that can help us understand the ways in which genes and Behavioral Genetics the environment—sometimes called nature and nurture—combine to inﬂuence behavior and mental processes (Moffitt, Caspi, & Rutter, 2005). Consider Mark and John, identical twins who were both adopted at birth because their parents were too poor to care for them. John grew up with a couple who made him feel secure and loved. Mark went from orphanage to foster home to hospital and, finally, back to his biological father’s second wife. In other words, these genetically identical people had encountered quite different environments. Yet, when they met for the ﬁrst time at the age of twenty-four, they discovered similarities that went beyond physical appearance. They used the same after-shave lotion, smoked the same brand of cigarettes, brushed with the same imported brand of toothpaste, and liked the same sports. They had joined the military within eight days of each other, and their IQ scores were nearly identical. How had genetic inﬂuences operated in two different environments to result in such similarities? Exploring questions such as this has taken psychologists into the ﬁeld of behavioral genetics, the study of how genes and environments work together to shape behavior and mental processes. They have already discovered that most behavioral tendencies can be inﬂuenced by many different genes but also by many environmental events and conditions, before and after birth. Accordingly, research in behavioral genetics aims to explore the relative roles of genetic and environmental factors in creating differences among people in personality, mental ability, mental disorders, and other phenomena. It also seeks to identify speciﬁc genes that contribute to hereditary inﬂuences. Some behavioral genetics research takes the form of experiments, mainly on the selective breeding of animals (Suomi, 2004). For example, Stephen Suomi (1999) identified monkeys whose genes predisposed them to react strongly or weakly to stress. He then mated strong reactors with other strong reactors and mated weak reactors with other weak reactors. Within a few generations, descendants of the strongreactor pairs reacted much more strongly to stressors than did the descendants of the weak-reactor pairs. Selective-breeding studies must be interpreted with caution, though, because it is not speciﬁc behaviors that are inherited. What is inherited are differing sets of physical structures and capacities that make certain behaviors more or less likely. These behavioral tendencies are often very speciﬁc, and they can be altered by the environment (Grigorenko, 2002). For example, when Suomi (1999) placed young, highly stress-reactive monkeys with unrelated “foster mothers,” he discovered that the foster mothers’ own stress reactivity ampliﬁed or dampened the youngsters’ genetically inﬂuenced behavioral tendencies. If stress-reactive monkeys were placed with stress-reactive foster mothers, they tended to be fearful of exploring their environments and had strong reactions to stressors. But if equally stressreactive young monkeys had calm, supportive foster mothers, they appeared eager to explore their environments and were much less upset by stressors than their peers with stress-reactive foster mothers. Research on behavioral genetics in humans must be interpreted with even greater care. Legal, moral, and ethical considerations obviously prohibit the selective breeding of people, so most research in human behavioral genetics depends on correlational studies, not controlled experiments. These usually take the form of family studies, twin studies, and adoption studies (Plomin, DeFries, et al., 2001; Rutter et al., 2001). In family studies, researchers look at whether close relatives are more likely than distant ones to show similarities in behavior and mental processes. If increasing similarity is associated with closer family ties, the similarities might be inherited. 38 1.8 Family and Twin Studies of Schizophrenia The risk of developing schizophrenia, a severe mental disorder, is highest for the siblings and children of patients with schizophrenia and lowest for those who are not genetically related to anyone with schizophrenia. Does this mean that schizophrenia is inherited? These results are consistent with that interpretation, but the question cannot be answered through family studies alone. Environmental factors, such as stressors that close relatives share, could also play an important role. Studies comparing identical and nonidentical twins also suggest genetic inﬂuence, but even twin studies do not eliminate the role of environmental inﬂuences. 50 Percentage schizophrenic FIGURE Chapter 1 Introduction to the Science of Psychology 40 30 20 10 Relationship to schizophrenic person Identical twin Child Nonidentical twin Niece/nephew Nontwin sibling Grandchild Spouse or general public For example, data from family studies suggest a genetic basis for schizophrenia, as Figure 1.8 shows. Remember, though, that a correlation between variables does not guarantee that one is causing the other. The appearance of similar disorders in close relatives might be due to environmental factors instead of, or in addition to, genetic ones. After all, close relatives tend to share environments, as well as genes. So family studies alone cannot establish the role of genetic factors in mental disorders or other characteristics. Twin studies explore the nature-nurture mix by comparing the similarities seen in identical twins with those of nonidentical twin pairs. Twins usually share the same environment and may also be treated very much the same by parents and others. So, if identical twins—whose genes are the same—are more alike on some characteristics than nonidentical twins (whose genes are no more similar than those of other siblings), those characteristics may have a signiﬁcant genetic component. TWINS AND BEHAVIORAL GENETICS Like other identical twins, each member of this pair has identical genes. Twin studies and adoption studies help to reveal the interaction of genetic and environmental inﬂuences in human behavior and mental processes. Cases in which identical twins who have been separated at birth are found to have similar interests, personality traits, and mental abilities suggest that these characteristics are signiﬁcantly inﬂuenced by genetic factors.