6. Behavioral Genetics I
Stanford
Summary
The field of behavioral genetics, while aiming to identify genetic components of behavior, has historically relied on flawed methodologies that often confuse environmental influences (especially prenatal and early-life experiences) and indirect genetic effects with direct genetic inheritance.
Key Takeaways
- Early Flaws: Initial approaches in behavioral genetics, such as observing traits "running in families" or being universal within a species, are fundamentally flawed because shared environment often confounds shared genes, making it impossible to disentangle genetic from environmental influences. 6:05
- Twin Study Criticisms: The classic method comparing identical (monozygotic) and fraternal (dizygotic) twins to infer genetic influence is undermined by the fact that identical twins are often treated more similarly and experience more shared environments than fraternal twins, even if raised together. 10:39
- Adoption Study Limitations: While adoption studies, like Kety's landmark schizophrenia research (showing 9% incidence with biological legacy vs. 1% general population), provided early evidence for heritable psychiatric traits, they are complicated by non-random placement of adoptees and significant prenatal environmental effects. 32:09
- Prenatal Environment's Profound Impact: Environmental factors do not begin at birth; prenatal experiences (e.g., maternal stress hormones, nutrient availability) can cause lifelong and even multi-generational changes in offspring's biology and behavior, like the Dutch Hunger Winter's metabolic programming leading to increased adult disease risk. 52:59
- Non-Mendelian & Lamarckian Inheritance: Genetic inheritance is not always a 50/50 split from parents (e.g., mitochondrial DNA is solely maternal) and can be influenced by imprinted genes. Furthermore, environmental factors can induce "Lamarckian" inheritance by disrupting transcription factor activity in eggs, leading to heritable non-expression of genes across generations. 1:18:27
- Indirect Genetic Effects: Many seemingly heritable behavioral traits are actually indirect genetic effects, where a heritable physical or physiological trait (e.g., height, feather color, pain sensitivity, ambiguity tolerance) causes individuals to be treated differently, leading to behavioral changes (e.g., extroversion, social rank, political affiliation). 1:23:34
- Epigenetic Mechanisms: Early life experiences, such as maternal care in rats (licking and grooming), can induce epigenetic changes that alter gene expression (e.g., for stress hormone receptors) throughout life and across generations, creating non-genetic, heritable behavioral patterns that can even be reversible by later experiences. 1:35:30
