The differential impact of birth order and IQ on sibling roles were examined with particular interest focused on achievement outcomes. Subjects were a stratified sample of 37 pairs of near-in-age siblings, all within the normal range in personality and IQ, but differing significantly in scores on the Slosson IQ Test. Results indicate that when the younger brother is intellectually more talented than his older brother, assignment of sibling roles appears to depend more on birth order socialization factors than on individual potential. (Author)