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Psychologists believe that by understanding how heredity and the environment affect behavior, they will be better able to help people with behavior problems. However, basic research is often performed using animals. Two important research methods used in animal behavior genetics are inbred strain comparisons and selective breeding (Plomin, DeFries, McClearn, & Rutter, 1997).
Researchers produce inbred strains by mating animals that are closely related genetically (such as brother–sister) over a number of generations. Eventually the offspring become more identical to one another genetically, and are then said to belong to an inbred strain. The inbred strains can be used to test whether genetic differences affect behavior. If two genetically different inbred strains of animals are reared in identical environments, any differences in behavior should be due to the genetic differences of the strains. For example, Badiani, Castellano, and Oliverio (1991) recently found that different inbred strains of mice react differently to stress. This suggests that genetic differences in neurotransmitter functioning may explain different types of stress reactions in mice. Inbred strains can also be used to test the influence of the environment on behavior. If groups of animals from the same inbred strain are reared in different environments, any behavior differences in the groups should be due to environmental influences (or more precisely, to the interaction of heredity with the environment). Another important animal research technique is selective breeding, in which similar animals are mated over many generations. Because they have similar genes, if you continue to mate the most similar animals, eventually you will obtain strains of animals that are different from the original population. In their 1965 book, Scott and Fuller explain that dogs have been bred selectively for ten thousand years, resulting in the wide variety of specialized breeds we have today. Wimer and Wimer (1985) review the types of animal behaviors that have been bred selectively, including activity, learning, emotionality, aggression, and even alcohol preference. In a classic selective breeding study, Robert Tryon (1940) selected rats for their ability to find their way in a maze. He mated the animals that made the fewest errors in finding their way in the maze (maze bright) together and the ones that made the most errors (maze dull) together. Then he mated the most similar offspring for twenty-one generations. After seven generations, Tryon had developed two genetically different lines of rats (maze bright and maze dull). Another classic behavior genetic study was conducted by Cooper and Zubek (1958). For their experiment, they used selectively bred rats who had been bred from the maze bright group and some from maze dull group. Since the maze bright and maze dull rats were reared in identical environments, the difference between the two groups could only be attributed to heredity. Cooper and Zubek placed the rats from both groups in either a restricted environment (in an empty cage with gray walls) or an enriched environment (in a cage with designs on its walls that contained objects such as ramps, mirrors, swings, balls, slides, and tunnels). When raised in restricted environments, the dull rats made many errors as usual, but the bright rats also made many errors. And when raised in the enriched environment, both groups made few errors. Since the genotypes did not change, it seems clear that the changes in behavior were caused by the changes in environment. Cooper and Zubek argued that heredity and environment always interact to produce final behavior. |
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