Kendler et al (2006)

• Aim:

  To investigate genetic prediposition to depression.
• Method: 

  correlation - one of the variables is relatedness and this was either 50% in the DZ twins or 100% in the MZ twins; the other variable was depression.
• Procedure

  
  

  • Sample: Over 42,161 twins from the Swedish twin registry
  • Twins assessed for MDD diagnosis
  • Concordance rates calculated for MZ and DZ twins, separated by gender
• Participants:

  • Included 42,161 twins from the Swedish Twin Registry.
  • 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
• 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.
• Limitations:

  1. Concordance rate for MZ twins not 100%, indicating environmental factors play a role.
  2. Twin studies assume equal environments for MZ and DZ twins, which may not always be true.
  3. Doesn't account for epigenetic factors that might influence gene expression.
  4. Correlational design limits causal conclusions about genetic influence.
• Strengths:

  1. Large sample size increases reliability and generalizability.
  2. Use of twin registry provides a representative sample.
  3. Comparison of MZ and DZ twins allows for estimation of genetic influence.
  4. Supports gene expression theory, acknowledging environmental factors.