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Identical (monozygotic) twins:

Alisha Janowsky

Created on October 2, 2025

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Transcript

Fraternal (Dizygotic) Twins

Identical (monozygotic) twins:

Occurs when a fertilized egg splits in two
Dizygotic twins (DZ) are from two fertilized eggs
Monozygotic twins (MZ) share 100% of their DNA
Like any siblings, DZ twins share 50% of their DNA

Critical Assumption

The Test

Research Evaluation

  • If genes influence personality, identical twins should be more similar than fraternal twins on personality traits
  • Both types of twins share similar environments (same family, same age, same schools)
  • Strengths
  • Weaknesses

Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International by Alisha Janowsky

ID vs. Fraternal Twins

ID Twins Raised Apart

Shared Environment Effect

  • ID twins are 2.5 times more similar than fraternal twins
  • Share the same environment but differ in genetic similarity
  • Extra similarity must be from genes
  • Translates to heritability coefficients of 40 - 50%
  • ID twins remain very similar
  • Never shared environment (besides the womb) but personalities are highly correlated
  • Powerful evidence of genetic influence
  • Personalities of adopted siblings raised together show almost no similarity
  • Shared family environments have little impact on personality
  • Shared family environment does influence things like values, beliefs, religious affiliations, etc.