Research on twins and unrelated persons living together indicates that, although differences in intelligence 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 environment on the similarity between relatives, and studies of environmental effects in other family constellations may be enlightening. In a sample comprising more 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 environmental factors have an impact on the intelligence of young adults. Analyses of subgroups defined by family size and parental education demonstrated that the effects of age difference are essentially independent of these factors.