We explore the effects of sibship density, i.e., the number of siblings closely spaced versus widely spaced, and sex composition, i.e., the number of sisters versus the number of brothers. Guided by two explanations previously put forth to account for the consistently found inverse relationship between sibship size and academic outcomes, the confluence model and the resource dilution hypothesis, we test whether these two structural parameters influence academic consequences. Analysis of the High School and Beyond data confirms a stronger negative effect of the number of siblings closely spaced than of the number of siblings widely spaced, an effect that endures into later adolescence even upon controlling for earlier test performance. Findings on sex composition, based on the National Longitudinal Survey of High School Seniors of the Class of 1972, are mixed. We find no effect of sex composition on test performance, but find a negative one on grade-point average. We conclude by highlighting the utility of decomposing sibship size into further components and of obtaining data that will facilitate such research efforts.