| A psychological research method that focuses on examining genetic and environmental influences on behavioral and medical characteristics. (Segal, 1990)
Twin research has been at the center of the nature vs. nurture debate for years. It examines such characteristics as IQ, alcoholism, personality type, depression, homosexuality, and weight in twin test subjects and compares them to control groups of fraternal twins and other siblings.
The results of these studies challenge the views of such psychologists as B.F. Skinner and Sigmund Freud: that individuals are as psychologically malleable as has been thought. They confirm that genetic predisposition has a role in determining individuality as well as factors of environmental influence and personal choice. In particular, twins are focused on that have been raised in separate homes. This factor allows psychologists to isolate the effect genes play on the adult.
Monozygotic (identical) twins have identical genotypes, as opposed to dizygotic (fraternal) twins and other siblings which share only 50% of their genes. Monozygotic twins occur when cells from a fertilized egg separate in an early cell division. The rate of monozygotic twins is 4-5 per 1,000 births worldwide. Dizygotic twins occur when two eggs are released and are fertilized by two separate sperm. They have the same relationship as any other siblings.
A particularly interesting characteristic to evaluate using twin studies is intelligence. Many factors have been shown to influence intelligence, but twin studies varify that genetics play an important role.
"Pairs of unrelated siblings of the same age, reared together from infancy (UST-SA), uniquely replicate the rearing situations of dizygotic (DZ) twins. These dyads offer a new behavioral-genetic design for examining genetic and environmental influences on behavior. An IQ intraclass correlation of .17 [Note from RainDropUp: A correlation of 1.00 indicates a perfect positive relationship, meaning that the two variables are directly related. A correlation does not necessarily indicate a causal relationship], based on 21 UST-SAs, is substantially lower than the correlations of .86, .60, and .50 reported for monozygotic (MZ) twins, DZ twins, and siblings, respectively. This finding supports an explanatory model of intelligence that includes genetic factors. The very modest IQ similarity between UST-SAs, despite their common rearing, suggest that the shared environment has a very small effect on intellectual development and supports the position that individuals respond to environments in ways consistent with their genetic predispositions. The results also challenge some critics' views that the behavioral resemblance of MZ twins is primarily a function of shared experience." (Segal, 1997)
Twin studies are a fascinating method of psychological research because of their ability to correctly isolate a characteristic and determine its impact. It is difficult to predict the exact influence of heredity due to various genetic patterns. However, twin studies are instrumental in offering evidence against the purely environmental model set forth by the Behaviorist school of psychology.
Works Cited:
Segal, N. L. (1997). Same-age unrelated siblings: A unique test of within-family environmental influences on IQ similarity. Journal of Educational Psychology, 89(2), 381-390.
Segal, N. L. (1990). The importance of twin studies for individual differences research. Journal of Counseling and Development, 68, 612-622 | 
