twins and genetics

Behavioral genetics is a field of research that investigates the relative effects of heredity and environment on behavior and ability (Plomin, 1997). Two of the primary methods used by behavioral geneticists are the twin study method, first used by Galton (1975) in his studies of heredity, and the adoption method.
In the twin study method, researchers studies identical twins (monozygotic twins) and fraternal twins (dizygotic twins) to determine how much they resemble each other on a variety of characteristics. Identical twins have exactly the same genes because a single sperm cell of the father fertilizers a single egg of the mother, forming a cell that then splits and forms two human beings-“carbon copies.” But fraternal twins are no more alike genetically than any two siblings born in the same parents. In the case of fraternal twins, two separate sperm cells fertilize two separate eggs that happen to be released at the same time during ovulation.
Twins, who are raised together, whether identical or fraternal, have similar environments. If identical twins raised together are found to be more alike than fraternal twins on a certain trait, then that trait is assumed to be more influenced by heredity. But if identical twins and fraternal twins from similar environments do not differ on a trait, then that trait is assumed to be influenced more by environment.
In the adopting method, behavioral geneticists study children adopted shortly after birth. By comparing their abilities and personality traits to those o their adoptive family members with whom they live and those of their biological parents whom they may have met, researchers can disentangle the effect of heredity and environment (Plomin et al., 1988).
Adoptive research has assembled the Minnesota Twin Registry, which in 1998 included over 20,000 twin pairs (Bouchard, 1998).
Probably the best way to assess the relative contributions of heredity and environment is to study identical twins that have been separated at birth and raised apart. Although it seems amazing, researchers have found that identical twins that are brought up in the same family are no more alike as adults that are identical twins who are reared apart. When separated twins are found to have strikingly similar traits, it is assumed that heredity has been a major contributor to those traits heredity, and the adoption method.
One of the most extensive investigation of twins raised in separate homes is the Minnesota Study of Twins reared apart, which over the past 20 years has studied hundreds of twin pairs who were separated early in life (Bouchard, 1994; Finkel et al., 1995). This study, like others of its kind, has consistently found such striking psychological and behavioral similarities between monozygotic twins that the important role of genes in personality development can no longer be denied.
Typical is the case of Oskar Stohr and Jack Yufe, identical twins born of a Jewish father and Christian mother in Trinidad in the 1930s. Soon after their birth, Oskar was taken to Nazi Germany by his mother to be raised as a Catholic in a household consisting mostly of women. Jack was raised as a Jew by his father, spending his childhood in the Caribbean and some of his adolescence in Israel.
On the face of it, it would be difficult to imagine two more disparate cultural backgrounds. And when the twins were reunited in middle age, they certainly had their differences. Oskar was married and a devoted union member; Jack was divorced and the owner of a store in southern California. But when the brothers met for the first time in Minnesota,

Similarities started cropping up as soon as Oskar arrived at the airport. Both were wearing wire-rimmed glasses and mustaches, both sported two-pocket shirts with epaulet. They share idiosyncrasies galore: they like spicy foods and sweet liqueurs, are absentminded, have a habit of falling asleep in front of the television, think it’s funny to sneeze in a crowd of strangers, flush the toilet before using it, store rubber bands on their wrists, read magazines from back to front, dip buttered toast in their coffee. Oskar is domineering toward women and yells at his wife, which Jack did before he was separated. [Holden, 1980]

Their scores on several psychological tests were very similar, and they struck the investigator as remarkably similar in temperament and