🔹 2. Genetic Similarities and Twin Studies

📘 Notes

Overview
Twin and family studies are classic tools to separate genetic from environmental influences. Monozygotic (MZ) twins share ~100% of their DNA, while dizygotic (DZ) share ~50%, allowing comparison of heritability estimates.

Key Study 1: Kendler et al. (2006)

• Aim: Investigate heritability of major depression.
  
• Method: Over 42,000 Swedish twins studied via registry data.
  
• Findings: Heritability of depression ≈ 38%; higher in women.
  
• Conclusion: Genetic factors moderately contribute to depression, but environment remains key.
  
• Evaluation:
  
  • 👍 Large, representative sample.
    
  • 👎 Correlational; cannot pinpoint causal genes.
    

Key Study 2: McGuffin et al. (1996)

• Aim: Examine concordance rates for depression.
  
• Findings: MZ = 46%; DZ = 20%.
  
• Conclusion: Genetic factors strongly implicated; environment still important.
  

Key Study 3: Scarr & Weinberg (1983)

• Adoption study on intelligence: adopted children resembled biological parents more in IQ → genetic influence significant.
  

💡 TOK Links How do probabilistic correlations challenge what it means to “know” something scientifically? Can statistical data truly explain human individuality?

🌍 Real-World Connections Twin registries used in studying addiction, personality, and schizophrenia. Policy implications for early screening and interventions.

❤️ CAS Links Create informative posters explaining twin research ethics.

🧪 IA Guidance Model twin correlations through small class surveys comparing siblings’ traits.

🧠 Examiner Tips Always explain why twin studies are used (control for genes). Avoid stating “genes cause behaviour” — always refer to interaction.