A correlation of +.71 would give a coefficient of determination of approximately .50 (or 50%). This means that of all the things that can affect the IQ of a person, we can account for about 50 percent of the variance by knowing the IQ of the person's identical twin. A correlation of +.30 yields a coefficient of determination of only 9 percent. So, of all the things that can affect a child's IQ, only 9 percent is explained by knowing the parents' income.