<P> Concordance, as used in genetics, usually means the presence of the same trait in both members of a pair of twins . However, the strict definition is the probability that a pair of individuals will both have a certain characteristic, given that one of the pair has the characteristic . For example, twins are concordant when both have or both lack a given trait . The ideal example of concordance is that of identical twins . </P> <P> A twin study compares the concordance rate of identical twins to that of fraternal twins . This can help suggest whether a disease or a certain trait has a genetic cause . Controversial uses of twin data have looked at concordance rates for homosexuality and intelligence . </P> <P> Because identical twins are genetically virtually identical, it follows that a genetic pattern carried by one would very likely also be carried by the other . If a characteristic identified in one twin is caused by a certain gene, then it would also very likely be present in the other twin . Thus, the concordance rate of a given characteristic helps suggest whether or to what extent a characteristic is related to genetics . </P>

A concordance rate of 50 percent indicates that