The invalid separation of effects of nature and nurture: Lessons from animal experimentation

Abstract

Introduction Several decades of research involving nonhuman animals have led many scientists to a developmental systems explanation of the origins of brain and behavior. Developmentalists recognize the importance of genetic variation for individual differences in behavior but also appreciate that the complex sequence of bidirectional, interacting causes makes it almost impossible to assign a definite role to the genotype unless a major gene can be identified. The prevalent model in human behavior genetics, on the other hand, presumes that heredity and environment are additive, separately acting causes whose contributions to any characteristic can be neatly separated statistically. This presumption is biologically unrealistic in view of all that is known today about the control of gene action and the interdependence of genetic and environmental effects. In this essay, we focus on lessons learned from animal experimentation that have special relevance for understanding individual differences among humans. We argue that a general theory of proximate causes of behavioral diversity is broadly applicable to all mammals and, in most respects, all vertebrates, but the theory is a developmental one, not one based on the invalid dualism of nature-nurture inherent in quantitative behavior genetics. Relevance of animal research Experimentation with animals is relevant to questions about genetic influences on human intelligence because remarkable plasticity of behavior is widespread among animal species, because quantitative genetic models of intelligence do not invoke any process that is uniquely human, and because there is a high degree of homology between all mammals at the genetic level and even with regard to the structure of the brain.