Family studies —studies that correlate degree of genetic relatedness among family members with degree of personality similarity—capitalize on the fact that there are known degrees of genetic overlap among family members. Parents are usually not related to each other genetically. However, each parent shares 50 percent of his or her genes with each of the children. Similarly , siblings share 50 percent of their genes, on average. Grandparents and grandchildren share 25 percent of their genes, as do uncles and aunts with their nieces and nephews. First cousins share only 12.5 percent of their genes.
If a personality characteristic is highly heritable, then family members with greater genetic relatedness should be more similar to each other than are family members with less genetic relatedness. If a personality characteristic is not at all heritable, then even family members who are closely related genetically , such as parents and children, should not be any more similar to each other than are family members who are less genetically related to each other .
If you have been following the logic of the ar gument thus far , you may have noticed a potential fla , or confound, in family studies—namely, members of a family who share the same genes also typically share the same environment. In other words, two members of a family might be similar to each other not because a given personality characteristic is heritable but, rather, because of a shared environment. For example, certain brothers and sisters may be similar on shyness not because of shared genes but because of shared parents. For this reason, results from family studies
alone can never be viewed as definitive. Finding that family members become increas ingly similar to each other as the percentage of genetic overlap increases is certainly compatible with a genetic hypothesis. But it cannot be regarded as conclusive evidence. A more compelling behavioral genetic method is that of twin studies.
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