Chapter 10: Heredity, Environment & Behavior

Rather than viewing nature and nurture as opposing forces, the text establishes a developmental pathway where inherited DNA interacts with prenatal and postnatal experiences to produce individual differences. While some conditions arise from specific chromosomal abnormalities or single mutant genes—such as Down’s syndrome, Huntington’s chorea, or color blindness—the vast majority of psychological traits follow a polygenic model. This model suggests that multiple genes combine with various environmental factors to determine an individual's predisposition toward certain characteristics, a concept known as the diathesis-stress theory. To disentangle these influences, researchers utilize family, twin, and adoption studies. For instance, comparing monozygotic twins, who share all their genes, with dizygotic twins, who share only half, reveals that traits like extroversion, sociability, and anxiety proneness have significant genetic components. Adoption studies further clarify these findings by showing that children often resemble their biological parents in areas such as criminal behavior, alcoholism, and sociopathy, even when raised in different environments. The chapter places significant emphasis on the role of heredity in major psychiatric disorders, particularly the functional psychoses. In the study of depression, a clear distinction is made between reactive depression, which is often tied to life events, and endogenous or bipolar disorders, which show much stronger evidence of genetic transmission. Schizophrenia receives the most detailed analysis, with evidence from global twin registries and cross-fostering research indicating that while environmental stressors can trigger the condition, genetic factors are the most potent and consistent predictors of risk. The text introduces the concept of a "schizophrenia spectrum," suggesting that a graded genetic predisposition exists across the population, where only those passing a certain threshold of combined genetic and environmental liability manifest the disorder. Ultimately, the chapter concludes that understanding behavior requires a balanced perspective that acknowledges the biological foundations of individuality without adopting a fatalistic view, as environmental interventions and treatments remain crucial regardless of genetic leanings.