Kendler et al (2006)

• Aim:

  To investigate the genetic contribution to Major Depressive Disorder (MDD) using twin study methodology.
• Research Method:

  Twin study comparing concordance rates for MDD between monozygotic (MZ) twins, who share nearly all their genes, and dizygotic (DZ) twins, who share about 50% of their genes.
• Procedure:

  • Sample: Over 40,000 twins from the Swedish twin registry
  • Twins assessed for MDD diagnosis
  • Concordance rates calculated for MZ and DZ twins, separated by gender. Monozygotic (MZ) twins (100% genetic similarity) and dizygotic (DZ) twins (50% genetic similarity) were studied.
• Results:

  • Higher concordance rates for MDD in MZ twins compared to DZ twins
  • Females: MZ twins had a 44% concordance rate vs 16% in DZ twins
  • Males: MZ twins had a 31% concordance rate vs 11% in DZ twins
  • Overall heritability of depression was estimated at 38%
  • Higher heritability of MDD was found in women compared to men
• Conclusion: 


  The study also supports the theory of gene expression. Even though the MZ twins had exactly the same genes, they had different life experiences and so would have a different epigenome - that is, different genes would be expressed. This could potentially explain why the concordance rate is not 100%. The higher concordance rate in MZ twins, especially in females, suggests a significant genetic component in the development of MDD.
• Strengths:

  • Robust Sample & Twin Design: A large twin sample provides strong evidence for genetic influence through clear differentiation between MZ and DZ twins.
  • Heritability Estimation: Offers quantitative insight into the proportion of MDD risk attributable to genetics.
• Limitations

  • Shared Environment Confound: It is challenging to completely separate the influence of shared environmental factors from genetic effects in twins.
  • Lack of Specificity: The study demonstrates genetic influence but does not identify specific genes or underlying mechanisms involved in MDD.