This chapter explores the potential benefits of a brain-behavior interdiscipline that incorporates a discipline, quantitative genetics—especially when they are integrated in the context of a fourth discipline, development. A new neuroscience area at the interface between biology and behavior has grown rapidly relative to more traditional areas of physiology: behavioral pharmacogenetics and genetic influences on drug responses. A brief review of quantitative genetic research on background electroencephalogram responses, evoked potential responses, autonomic activity, hemispheric lateralization, behavioral pharmacology, neurophysiology and neuroanatomy. Application of multivariate genetic-environmental analysis to longitudinal data has much to offer in developmental studies of brain, behavior, and their relationships. The analysis of age-to-age correlations provides an empirical approach to assessing genetic change as well as continuity during development. The merger of quantitative genetics and molecular genetics can capture the strengths of each and has great potential to advance our understanding of the complex phenotypes of brain and behaviour.