This study aims to identify specific genes important for cognitive functioning in the normal range. In a large sample of twins for whom we have varied measures of cognition we will conduct genome-wide analyses. Genome-wide scans capture common DNA variation at a level previously not possible. We expand our test battery to include a novel measure of cognition, relational processing, and examine how much variation is due to genes. The identification of genes for cognition will transform our understanding of brain functions that affect human intellect. This may have future applications for treating cognitive difficulties arising from brain dysfunction.