Oscar and Jack are identical twins, separated when they were babies by their parent's divorce. Jack was brought up by their Jewish father in Trinidad, Oscar went to live with their mother's family in Czechoslovakia. In fact Oscar did not know he was Jewish until Jack found him in Germany in 1954. Jack is proud to be a Jew, but Oscar still does not like to talk about his Jewish heritage. Yet the twins are similar in many ways. They both like spicy food and sweet drinks. In school they both did well in sports but poorly in mathematics. Are thoughts and behavior determined by heredity or by environment? Social scientists have long been interested in this question; the results of “twin studies” are particularly interesting to them. Twin studies are of the similarities and differences between twins. There are two types of twins: identical twins, who look exactly the same because they have identical genetic characteristics, and fraternal twins, who have different genetic characteristics. In a 1937 study, Newman, Freeman, and Halzinger found that identical twins are more similar in height and weight than fraternal twins. Because identical twins have the same genetic characteristics, the conclusion was that size is determined more by heredity than by environment. In 1962, James Shields compared the height, weight, and intelligence of identical twins who were brought up together ( in the same environment ) with those of identical twins brought up apart ( in different environments ). He found that although the differences were small, identical twins who were brought up together were more similar than those brought up apart. Shields concluded that height, weight, and intelligence are largely determined by heredity. However, because of the differences between identical twins brought up together and those brought up apart, these characteristics may be partly determined by environment. In the case of Oscar and Jack, heredity seems to determine physical and mental abilities, tastes, and behavior. Environment seems to determine thought.