Twin Studies And The Genetic Objection
Twin Studies and the Genetic Objection: A Critical Analysis
Twin studies are frequently cited as evidence for genetic determinism of behavior. This article examines what twin studies actually show, why they are often misinterpreted, and why language acquisition provides a definitive counterexample to genetic behavioral determinism.
The Objection
The genetic objection to environmental theories of behavior takes this form:
Twin studies show that identical twins raised apart are remarkably similar in personality and behavior. Since they share genes but not environment, this proves behavior is primarily genetic.
What Twin Studies Actually Show
The 50-Year Meta-Analysis
Polderman et al. (2015) conducted a comprehensive meta-analysis of twin studies covering 17,804 traits from nearly 14.6 million twin pairs. Key findings:
- Average heritability across all traits: 49%
- This means environment explains 51% of variance—slightly more than genetics
- For psychopathology specifically, shared environment accounts for 10-30% of variance (Burt, 2009)
The claim "twin studies prove genetic determinism" misrepresents findings that show genetics and environment contribute roughly equally.
The Minnesota MISTRA Study: The Critical Finding
The Minnesota Study of Twins Reared Apart (MISTRA) is the most famous twin study, often cited to support genetic determinism. However, the researchers' own conclusion is frequently ignored:
A plausible hypothesis emerged from the findings that genetic differences affect psychological differences largely indirectly, by influencing the effective environment of the developing child. (Bouchard et al., 1990)
This is profound: genetics operates through environment, not instead of it. Genes influence temperament → temperament influences how caregivers respond → caregiver responses create the effective environment → environment shapes behavior.
The causal chain runs through environment. Genes are upstream; environment is the proximate cause.
The Language Proof
The Smoking Gun
Language acquisition provides definitive proof that complex behavioral patterns can be 100% environmentally determined:
- No one is genetically predisposed to speak English: A child of Chinese parents raised in Australia speaks English natively. A child of English parents raised in China speaks Mandarin natively.
- Language is universal: Every neurotypical human learns the language(s) of their environment, regardless of genetic ancestry.
- Language is extraordinarily complex: Phonology, morphology, syntax, semantics, pragmatics—language is among the most sophisticated cognitive achievements humans exhibit.
- Environment is necessary: Genie Wiley, isolated from language until age 13, never fully acquired language despite intensive intervention. No environmental exposure → no language.
The Logical Extension
If language—a behavior of extraordinary complexity involving thousands of rules applied in real-time—is 100% environmentally determined, why would we assume simpler behaviors (emotional responses, aggression patterns, social scripts) are genetically hardwired?
The burden of proof shifts. To claim a behavior is "genetic," one must demonstrate it is fundamentally unlike language—that it emerges regardless of environmental exposure. This burden has not been met for most behavioral patterns.
Problems with Twin Study Methodology
The Equal Environments Assumption
Standard twin studies assume that identical and fraternal twins experience equally similar environments. If identical twins are actually treated more similarly (dressed alike, same classroom, similar peer groups), this inflates heritability estimates.
Research suggests this assumption is often violated: identical twins do experience more similar environments than fraternal twins.
Selective Placement in Adoption
Twins "reared apart" are often placed in similar environments. Adoption agencies historically matched children to families similar to their biological parents. This means "separated" twins may share more environmental similarity than assumed.
Traditional twin models attribute twin similarity to either:
- Genetics (G)
- Shared environment (C)
- Non-shared environment (E)
But this trichotomy is too simple. Gene-environment correlation means genetic factors influence environmental exposure:
- Children who are genetically more active evoke more active parenting
- Children who are genetically more difficult evoke harsher responses
This correlation inflates heritability estimates while deflating environmental estimates.
Reframing the Twin Data
What Identical Twins Reared Apart Actually Demonstrates
When identical twins raised apart show similar personality traits, this can be interpreted as:
Genetic Interpretation: Genes directly cause personality.
Environmental Interpretation: Similar genes → similar temperament → similar environmental responses evoked → similar developmental environments → similar personality.
Both interpretations are consistent with the data. But the second interpretation explains more phenomena, including:
- Why intervention can change behavioral patterns
- Why behavioral patterns transmit culturally across genetically unrelated groups
- Why language is 100% environmental despite being cognitively complex
The Immigration Test
A natural experiment exists in every multicultural society:
If behavior were primarily genetic, children of immigrants should behave like people in their ancestral homeland.
If behavior were primarily environmental, children of immigrants should behave like people in their current environment.
Reality: Third-generation immigrants are behaviorally indistinguishable from the general population of their current country, regardless of ancestral origin. Same genetic ancestry → different behavioral patterns → environment is the dominant factor.
The Nuanced Position
We do not claim genes are irrelevant to behavior. Genes influence:
- Learning rates: Some individuals learn certain patterns faster
- Sensitivities: Some are more affected by environmental inputs
- Temperamental predispositions: Some are naturally more reactive or inhibited
- Risk factors: Some have genetic vulnerabilities that environment can trigger or buffer
But genes provide the substrate, not the content. A child with genes that facilitate fast learning will learn whatever their environment provides—whether that is English or Mandarin, anger or equanimity, trust or fear.
Environment determines what is learned. Genes influence how easily.
Implications
For Criminal Justice
If behavioral patterns are learned (like language) rather than genetically determined, then:
- Punishment for "choosing" bad behavior addresses the wrong level of causation
- Rehabilitation (retraining) should be the focus
- Prevention (improving developmental environments) is the most effective intervention
For Mental Health
If emotional patterns are learned (like language), then:
- "Anger management issues" means "learned anger as the appropriate response"
- Treatment should provide new models for observational learning
- Environmental intervention may be more effective than individual therapy
For Social Policy
If behavior is primarily environmental, then:
- Investment in early childhood environments yields highest returns
- Genetic screening for "criminal" or "mentally ill" genes is scientifically unsound
- Social determinants of health are primary targets for intervention
Conclusion
Twin studies do not prove genetic determinism of behavior. They show genetics and environment contribute roughly equally, with genetics operating through environmental mechanisms. Language acquisition—100% environmental, extraordinarily complex—proves that the most sophisticated human behaviors need not have any genetic component in their content, only in their capacity to be learned.
The question is not "nature vs. nurture" but "which behaviors are like language (100% environmental content) and which are not?" The burden of proof lies with claims of genetic determinism.
See Also
References
- Bouchard, T. J., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science, 250(4978), 223-228.
- Burt, S. A. (2009). Rethinking environmental contributions to child and adolescent psychopathology: A meta-analysis of shared environmental influences. Psychological Bulletin, 135(4), 608-637.
- Polderman, T. J. C., et al. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7), 702-709.
- Turkheimer, E. (2000). Three laws of behavior genetics and what they mean. Current Directions in Psychological Science, 9(5), 160-164.