Many define psychology as a biological science and emphasize brains and genes as major determinants of behavior. Instead, it is argued here that psychology is a unique biopsychosocial science able to stand on its own. Biogenetic processes are indeed relevant but are simply participating, not causal, factors in behavioral origins. Long neglected by biologists and social scientists, the importance of developmental processes is emphasized. The author takes issue with behavior geneticists and argues that development is bidirectional—internal and environmental phenomena influence behavior—probabilistically. The author favors a relatively new model with roots in ideas from contemporary physics: emergence and self-organization—“relational developmental systems.”