Most of the evidence bearing on the inheritance of intelligence comes from studies correlating IQs between persons of various degrees of genetic relationship in general, the closer the genetic relationship, the more similar the tested intelligence.

The average correlation between the IQs of parents and their natural children is 50s; between parents and their adopted children the correlation is about .25. Identical twins, because they develop from a single egg, share precisely the same heredity; the correlation) between their IQs very high-about .

The IQs of fraternal twins (who are genetically no more alike than ordinary siblings, since they develop from separate eggs) correlate about .55. Even when identical twins are reared apart in different homes their IQs correlate higher, .75, than those of fraternal twins reared together adopted children resemble their natural parents more than their adopted parents in intellectual ability, their IQs are higher than would be predicted on the basis of their natural parents’ ability. An improved environment makes a difference.