#211 — The Nature of Human Nature

Episode Summary

Executive Summary: Sam Harris speaks with behavioral geneticist Robert Plowman about how DNA shapes psychology, personality, intelligence, and psychopathology. They explain heritability, the limits of shared-family nurture, why many environmental effects are actually nonshared or genetically mediated, and how polygenic scores and quantitative traits challenge simplistic disorder models and the blank-slate view.

Main Topics: Origins of behavioral genetics and Plowman’s career (Priority: 4/5): Plowman describes accidentally discovering behavioral genetics at the University of Texas in the early 1970s and how the field’s taboo status shaped his work. Heritability and what it really means (Priority: 5/5): The conversation clarifies that heritability is a descriptive statistic about differences in a population, not a statement about immutability or individual destiny. Nature, nurture, and non-shared environment (Priority: 5/5): They discuss how twin/adoption studies show that shared family environment explains surprisingly little, while non-shared and stochastic influences matter more than people assume. Genes shaping environments and the nature of nurture (Priority: 5/5): Many apparent environmental effects reflect genetic differences influencing behavior, preferences, and social selection, such as reading, peer choice, and discipline. Polygenic inheritance, epigenetics, and complex traits (Priority: 5/5): Plowman explains that most behavioral traits are influenced by thousands of small genetic effects, not single genes, and that epigenetics does not undermine DNA-based inheritance. Dimensions vs. disorders in psychology and psychiatry (Priority: 4/5): They argue that conditions like schizophrenia and depression are better understood as quantitative extremes on continuous traits rather than discrete disease categories. Political and ethical implications of genetic research (Priority: 4/5): The discussion addresses controversy over group differences, the dangers of simplistic political readings, and why genetics does not negate fairness or equal moral worth.

Key Arguments: DNA explains a large share of psychological and behavioral differences, often around half the variance in traits studied. Heritability is population-specific and does not mean a trait is fixed, inevitable, or purely genetic in every individual. Shared family environment is much less important than commonly assumed; adoptive parents usually do not make unrelated children more similar on traits like BMI. Many correlations attributed to parenting or environment are partly or largely due to genetic differences between children and parents. Non-shared environment exists, but much of it appears idiosyncratic or stochastic rather than a stable, controllable family factor. Twin and adoption studies converge on the conclusion that psychology is substantially heritable despite different assumptions and methods. Epigenetics affects gene expression but does not replace or invalidate inherited DNA sequence differences as the basis of heritable variation. Complex traits are polygenic and pleiotropic: thousands of tiny genetic effects influence many outcomes, making single-gene models misleading. Psychiatric categories are often better treated as continuous dimensions, because genetic liability is distributed across the whole population. Group-difference research is politically explosive and methodologically weaker than research on individual differences; Plowman avoids it for those reasons.

Data Points: Psychological trait variance explained by genetic differences: about 50% on average - Plowman’s summary of the first law of behavioral genetics across traits Body mass index heritability: about 70% - Estimate for northern European populations in the discussion of BMI Parent-child BMI correlation: about 0.3 - Biological parents and children resemble each other on BMI Adoptive parent-child BMI correlation: 0 - Unrelated adopted children do not resemble adoptive parents on BMI Adopted child-birth parent BMI correlation: about 0.3 - Genetically related birth parents predict adopted children despite no shared upbringing Sex differences in math/verbal ability variance: about 1% - Example used to show that between-group differences are tiny relative to within-group variance Human DNA similarity: 99% identical - Common DNA shared by humans as a species DNA sequence difference between people: about 1% - The small fraction of DNA variation that contributes to individual differences Genome size mentioned in discussion: 6 billion base pairs total; 3 billion from each parent - Used to explain inherited DNA and human similarity Body mass index polygenic prediction: about 10% of variance - Aggregate prediction from many DNA differences Height polygenic prediction: about 25% of variance - Aggregate prediction from many DNA differences Single BMI-associated variant effect: about 1% of variance - One of the largest early GWAS effects described as a major finding Pounds difference from one BMI-associated allele: 3 pounds heavier per A allele; 6-pound gap between TT and AA - Illustration of a specific DNA variant’s effect on body weight Number of single-gene disorders: thousands (roughly 7,000 to 10,000 mentioned) - Mendelian disorders discussed as distinct from complex traits Rare single-gene disorder frequency: one in 100,000 to one in 500,000 - Illustrating that monogenic disorders are rare compared with polygenic traits

Pivotal Quotes: "it might seem unbelievable today, but 30 years ago, it was dangerous professionally to study the genetic origins of differences in people's behavior" — Sam Harris (quoting Plowman’s book introduction): Sets up the historical stigma around behavioral genetics "the vast majority of the variance on these traits is within groups rather than between groups" — Robert Plowman: Explaining why group-difference debates are less informative than individual-difference research "what we inherit are the DNA differences" — Robert Plowman: Clarifying that heredity concerns sequence variation, not acquired environmental changes

Implications: For listeners, the takeaway is that human differences are strongly shaped by DNA but not destiny; parenting and policy matter, yet often less than assumed. Future psychology and psychiatry will likely move toward polygenic, dimensional models that replace simplistic blame, cure, and blank-slate narratives.