The relationship of intelligence and achievement to brith order, sex of sibling, and age interval.

Mother-Child and Sibling-Sibling Interactions on a Problem-Solving Task

“…However, the interaction effect of the treatment factor with sibling sex is of considerable interest. Earlier studies of the effects of sibling structure on the child's abilities and achievement (Koch 1954;Schoonover 1959) suggested that an older male sibling was somehow more stimulating to the younger child than was an older female sibling; the present results would indicate that the situation may be more complex, where the mother tends to give help herself to the child with an older brother but tends to delegate the help where the child has an older sister. When the helper was an older sibling, older sisters explained more, gave more feedback, and more total verbalization about the task than did older brothers.…”

Selected Factors Influencing Over- and Under-Achievement in Young School-Age Children

“…Another investigator (Schoonover, 1959) utilized longitudinal data to analyze sibling performance on intelligence tests of achievement. No significant differences were found between older and younger siblings in intelligence or achievement as measured by deviations from the norms for chronological age.…”

School Achievement of First- and Second-Born Siblings

“…With some exceptions (e.g., Cicirelli, 1967;Schoonover, 1959), evidence of first-born superiority in scholastic achievement has been reported consistently in the literature. The present study, adopting a different design, served to confirm this finding for school-age populations.…”