Many human characteristics have been studied with the goal of determining the relative contributions of heredity and environment. The methods employed are, of course, necessarily less direct than the technique of selective breeding-and the results, accordingly, are sometimes ambiguous, Family studies often indicate that a particular trait occurs more frequently among parents, siblings, and other relatives in some family lines than in the population at large. However, such data do not make clear whether the cause of such Grinning in families” is the genetic similarity of close relatives, or the fact that their environments are similar, or both.
Twin studies try to provide an answer by comparing, for the trait in question, the similarity of identical twins with the similarity of fraternal twins. Since identical twins have 100 percent of their genes in common. While fraternal twins have only about 50 percent, identical twins should be more alike than fraternals if the trait is genetically determined. However since it is known that identical twins experience more similar environments than do franternals, environment is not in fact held constant when the two types of twins are compared. Finally, adoption studies focus upon children given up for adoption at a very early age and thus reared without contact with their biological relatives. The aim of an adoption study is to separate genetic from environmental variables; such studies depend upon the assumption that the placement of children into adoptive homes has been essentially random. Complications arise if for example the biological children of “superior” parents tend to be placed into equally ”superior” adoptive homes.
Twin and adoption studies have focused on a number of physical mental.and behavioral trait. The results of such studies will be reported and critically evaluated where they are relevant throughout my articles.
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