(1) Environmental differences between different families,
(2) Genetic differences between families,
(3) Environmental differences between individuals within families, and
(4) Genetic differences between individuals within families. Carrying out the appropriate calculations reveals that about three-fourths of the total variance of IQ scores is associated with genetic factors and one fourth with environmental factors plus errors of measurement, parents of a certain gene type can influence their offspring both by direct genetic transmission and by the kind of environment they provide. Thus, brighter parents may not only “brighter genes” but also a more intellectually stimulating environment.