Studies relating intellectual performance to birth order report conflicting results, some finding intellectual scores to increase, others to decrease with birth order. In contrast, the relationship between intellectual performance and family size is stable and consistently replicable. This birth order puzzle is resolved by means of a confluence model that quantifies the influences upon intellectual growth arising within the family context. At the time of a new birth, 2 opposing influences act upon intellectual growth of the elder sibling: (a) his or her intellectual environment is "diluted" and (b) he or she loses the "lastborn's handicap" and begins serving as an intellectual resource to the younger sibling. Since these opposite effects are not equal in magnitude, the differences in intellectual performance among birth ranks are shown to be age dependent. While elder children may surpass their younger siblings in intellectual performance at some ages, they may be overtaken by them at others. Thus, when age is taken into consideration, the birth order literature loses its chaotic character and an orderly pattern of results emerges. (59 ref) (PsycINFO Database Record (c) 2016 APA, all rights reserved)