In the science of behavioral development, we want to understand how the complete behavioral repertoire of an individual develops along trajectories (starting from conception and proceeding through differences in forms and processes across the lifespan) that yield both individual differences and species-typical similarities. For some decades, the conceptual frame for examining such questions was that behavior developed from an organism-environment interaction. The task for the developmental scientist was to demonstrate that both organism variables (e.g., genes, hormones, brain processes) and environmental variables (e.g., rearing conditions, social conditions, training) contributed to the developmental expression of some behavioral trait (e.g., memory, anxiety, parental care). Demonstrating that individual differences in the level of some hormone, presence of some genetic factor, or the activation of some brain area were responsible for differences in some behavioral trait (e.g., parental care) was always qualified by the recognition that environmental factors also played a role. Similarly, although rearing conditions, social influences, or teaching conditions were demonstrated to produce differences in the behavioral trait (e.g., parental care), these demonstrations were qualified by recognition that organism factors also played a role.Essentially, there were two methods for examining such interaction: 1) hold some aspects of a group of individuals constant (e.g., their genotype, hormonal condition) while varying some aspect of their environment; 2) hold some aspects of the environment constant (e.g., general rearing conditions) while varying some aspect of the organism (e.g., genotype). More complex designs would permit statistical analysis that would partition the phenotypic variability among individuals in the groups into those parts that were related more strongly to the