Intelligence Correlations Between Brothers Decrease With Increasing Age Difference Evidence for Shared Environmental Effects in Young Adults

ABSTRACT—Research on twins and unrelated persons liv-ing together indicates that, although differences in intel-ligence between persons are caused by both genetic and environmental factors, similarities—and especially those between adults—are mostly or exclusively due to shared genes. However, twin and adoption designs have been criticized for incorrectly estimating the effects of the en-vironment on the similarity between relatives, and studies of environmental effects in other family constellations may be enlightening. In a sample comprisingmore than 334,000 pairs of brothers (ages 18–21 years) who were tested at the mandatory conscription for military service in Norway, correlations between brothers ’ intelligence-test scores decreased with increasing age difference between the brothers. This result indicates that family environmenta