Sibling constellation factors were related to measures of creativity (Minnesota Tests of Creative Thinking), IQ, reading, arithmetic, and language achievement in a sixth-grade population using analysis of variance. In the first portion of the study, family size was found to be unrelated to ability and achievement (N = 140). The main portion of the study investigated birth order, sex, sex of sibling, and age difference in the 2-child family (N = 80), and found that verbal creative abilities, reading, and arithmetic achievement were enhanced for Ss with siblings of like sex close in age. In the final portion of the study, birth order was found to be unrelated to ability and achievement in 3- and 4-child families (N = 144), and sibling sex was found to be important in the 3-child family where Ss with 2 brothers had poorer IQ and reading scores than Ss with at least one sister (N = 96). Students of child development have long been interested in the implications which a child's position among his siblings has for his subsequent intellectual and personality development. It is the purpose of this paper to explore the relations which may exist between measures of cognitive development such as IQ, creative abilities, and academic achievement, and children's sibling constellations as represented by family size, birth order, sibling sex, and age difference between siblings.