Twin Studies and Behavior

Twin Studies and Behavior

• Twin studies investigate the correlation between genetics and behavior by examining twins who share varying degrees of genetic similarity.
• Monozygotic (MZ) twins develop from a single fertilized egg that splits; thus, they have identical genes (and are always the same sex).
• Dizygotic (DZ) twins develop from two separate fertilized eggs; they share about 50% of their genes (like any other siblings) and may be of different sexes.
• Twin studies compare MZ and DZ twins, not twins with non-twin siblings, to pinpoint genetic influences on behavior,
• MZ twins share 100% of their genes, while DZ twins share a maximum of 50%.
• Studying concordance rates helps reveal the influence of nature (genetics) versus nurture (environment).
• If MZ twins have identical behavior despite shared environments, genetics likely plays a minimal role in that behavior.
• If MZ and DZ twins have different behavior patterns, genetic differences play a larger role.
• The concordance rate measures the probability that both twins in a pair exhibit the same trait.
• 100 percent concordance indicates all pairs share the trait; 0 percent indicates none do.
• High concordance rates in MZ twins, compared to DZ twins, suggest a strong genetic component to a trait.
• Eye color is often given as an example where genetics largely determine the trait.

Concordance Rate Example: Criminality

• A Danish twin study (Christiansen, 1977) examined criminality in 3,586 twin pairs born between 1881-1910.
• The study's results showed a higher concordance rate for criminality in MZ twins (35%) than DZ twins (13%).
• This suggests a genetic influence on criminality, but also an environmental influence given the concordance rate wasn't 100% for MZ twins.
• A 100% concordance in MZ twins, for criminality would suggest a purely genetic basis.