The research described in this article addressed the question of why siblings commonly have different developmental outcomes despite their common beginnings. The studies analyzed behavioral development, especially through examination of deviant behaviors and intellectual development, by tracing siblings' different life histories. The work is based upon a theoretical framework that attributes behavioral variation to causal sources that define composite variance components. These variance components are composite because a variety of specific influences can unite to produce variation of a particular type. The article is divided into three sections that review the research program and findings. The first section focuses on using sibling and twin studies to attribute behavioral variation to composite variance components used in behavior genetic research. The second section discusses the issue of comorbidity--traits that are related in their familial co-occurrence. The third section describes specific sources of composite variance components. The chapter closes with a look at unanswered questions and directions for future work. Contains 37 references. (SD)