It has long been recognized that every individual is a product of genetic influences and the impact of the environment in which he or she was raised. In the 19th century, it was suggested that the study of twins might point out resolution of this question. Since identical twins which produced from the same egg have identical genetic patterns and the heredity factor could be considered as constant.

In the late 1970’s, the behavioral study of twins established a new impetus. They discover several especially striking pairs of identical twins—children separated by family at birth and rose in totally different environments. One of the most dramatic cases was Jack Yufe and Oscar Stohr. They are separated by their parent divorce in 1932, while they were still infants. Yufe was raised in Trinidad as a Jewish, studied Hebrew, and lived in Israel for five years. While Stohr was raised as a Roman Catholic in Czechoslovakia, underwent a Nazi-dominated education during the German occupation, and was strongly anti-Semitic. It appears that their personalities differ sharply. Yufe is a relatively supple individual, generally liberal in his political and social attitudes. While Stohr is fairly firm. There are, however, surprising and bizarre parallels, i.e. both of them like and dislike almost the same foods; both are weak in mathematics; both selected the same shade of green as their favorite color.