Twin Studies & Heritability in Criminal Behaviour

Twin Study Models
  • Traditional design: Compare monozygotic (MZ / identical) twins with dizygotic (DZ / fraternal) twins.
    • Logic:
    • Hold the environment as constant as possible.
    • Allow genetic similarity to vary (MZ share ≈ 100%100\% of segregating genes; DZ share ≈ 50%50\%).
    • Outcome variable discussed in this clip: criminal behaviour (measured either through arrest/conviction records or self-report instruments).
Criticisms of the Traditional MZ–DZ Comparison
  • Objection: The environment is not truly held constant.
    • MZ twins look alike ➔ tend to:
    • Be placed in more similar environments (same classroom, same peer group, same clothes).
    • Receive more similar social feedback (teachers, parents, peers treat them as a unit).
    • DZ twins usually appear less alike, so environmental overlap is often lower.
  • Implication: Higher concordance in MZ twins might be inflated by environmental similarity, not only by genes.
Reverse Strategy: Holding Genes Constant, Varying Environment
  • Solution: Study MZ twins raised apart.
    • Genes held constant (still ≈ 100%100\% shared DNA).
    • Environments differ—sometimes dramatically (different households, socio-economic status, neighborhoods).
Practical Challenges of the Raised-Apart Design
  • Rarity: Separation of identical twins at birth is uncommon.
  • Sample sizes: Many studies manage samples of only 5–6 twin pairs.
    • Statistically underpowered but still informative as concept proofs.
Core Findings from MZ-Apart Studies
  • Concordance rate for criminal behaviour ≈ 50%50\%.
    • Meaning: If one twin displays criminal behaviour, the co-twin raised in a different environment shows the same outcome about half the time.
  • Heritability coefficients (proportion of variance in criminal behaviour attributable to genetic factors):
    • Range reported: 0.28≤h2≤0.450.28 \leq h^{2} \leq 0.45.
    • Similar magnitude to estimates derived from the classic MZ–DZ design.
Synthesis of Both Strategies
  • Regardless of whether researchers:
    • Compare MZ vs. DZ twins reared together, or
    • Examine MZ twins reared apart,
  • Convergent conclusion: A genetic component contributes to criminal behaviour.
    • Neither method shows heritability h2=1h^{2} = 1, so environmental influences remain substantial.
Conceptual & Practical Implications
  • Research methodology: Demonstrates value of complementary designs to tease apart gene–environment confounds.
  • Policy/ethical angle: Acknowledging partial heritability prompts nuanced approaches—e.g., early intervention targeting at-risk environments rather than genetic determinism.
  • Caveats: Small samples and possible unmeasured post-natal contact between separated twins still limit definitive claims.