Twin Studies
Because monozygotic or identical twins have identical genes, they have been widely employed in medical and psychological research. The twin method compares sets of monozygotic twins to control sets of dizygotic or fraternal twins who have been raised in the same family environment to examine the relative importance of heredity over the environment in the development of certain diseases, in the formation of the personality and sexual orientation, and in intelligence. Through twin studies, researchers have, for example, demonstrated a strong genetic component in the development of schizophrenia, alcoholism, depression, obesity, and in the inclination to infectious diseases such as tuberculosis.
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The 19th-century behaviorist Sir Francis Galton first noted in 1875 that “twins have a special claim upon our attention” because “their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives.” Since Galton’s conclusion, twin studies have been a powerful if problematic tool in the hand of geneticists. Classical twin studies claim that any excess likeness in a given trait between identical twins when compared to fraternal twins, who only share 50% of the genes, should be due to genes rather than environment.
More contemporary twin studies have also tried to establish the extent of the impact of a person’s common surroundings (such as the family) and unique events in one’s life on a trait. A founding assumption of twin studies is random mating, the idea that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. Otherwise, fraternal twins could share more than 50% of their genes. A second important hypothesis of twin studies is the argument that fraternal and identical twins raised in the same homes experience equally similar environments.
Twin studies have had to face numerous challenges. Some researchers have concluded that people are more likely to select mates with similar levels of intelligence, making the application of the discipline to the study of intelligence particularly problematic. In addition, twin studies do not acknowledge the possible interactions between genes and environments, but tend to consider their influence on a trait separately. Twin studies, however, have also been capable of taking into account several objections and improving research methodologies, which have expanded to include examinations of twins’ extended families, longitudinal studies, and other variations. In the age of genomes and molecular genetics, the future of twin research seems to depend on its ability to combine its traditional methodologies with molecular genetics research.
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