Nature-nurture is unfit to account for the seamless co-determination of behavior by biological evolution and culture. The environment is the shaping causal factor both in evolutionary history of species and populations (phylogeny) and in the lifetime history of the organism (ontogeny). Thus evolutionary biology, studying how the environment shapes traits and behavior through the evolutionary history of the species, and psychology of learning, studying how the environment shapes and the behavior of an organism through its lifetime, can be considered compatible causal categorical accounts of behavior. Radical behaviorism, a philosophy of psychology which holds behavior to be governed by phylogenetic and ontogenetic contingencies, is considered. I find that radical behaviorism, though a favorable approach to psychology, is inconsistent with some important core-principles in modern evolutionary biology. A Neo-Darwinian radical behaviorism is proposed. This facilitates a better account of how phylogeny and ontogeny seamlessly co-determine human behavior. Furthermore, a modern synthesis for ontogeny is proposed, modeled after the modern synthesis between Mendelian genetics and Darwinian evolutionary biology. This solves the classical debate about whether behavior is caused by mental mechanisms or contingencies of reinforcement and punishment. The brain, encapsulating mental states, may be considered proximately causing behavior, while phylogenetic and ontogenetic contingencies ultimately cause behavior. I draw full circle by returning to biological evolution and culture. I argue that the proposed neo-Darwinian radical behaviorism enables a culture theory that both does justice to the autonomous nature of culture and cultural evolution, while seamlessly grounding culture in modern evolutionary biology. Finally, These insights imply a broad framework in which to define the causal and explanatory significance of key scientific fields within biology, psychology and anthropology — a philosophy of interdisciplinary science of behavior.