Genetic similarities/kindship studies

gentic similarities measures the proportion of shared genes between individuals to determine the biological basis of behavior

A research method comparing identical and fraternal twins to estimate the influence of genes and environment.

Monozygotic (MZ) twins
identical twins who share 100% of their genes.
Dizygotic (DZ) twins
Fraternal twins who share about 50% of their genes.

Genetic similarity studies require the use both identical and factual twins to measure behavior. If MZ twins show higher similarity in a behaviour than DZ twins, this suggests a genetic contribution to that behaviour.

Kendler et al (2006)

investigated the genetic and environmental influences on major depressive disorder

a mental health condition characterized by persistent low mood, lack of pleasure, and somatic symptoms

Correlation study (interviews + twin study)
used 15,493 twin pairs from Sweden's twin registry
Partipents were assesed via telephone interviews for lifetime major depression.

8056 twins met crtieria for a dignosis of MDD at some point in their life.
Life time MDD was also more common in female twin pairs compared to males
The concordance for life time major depression was also higher in Monozygotic twins compared to Dizygotic twins
This indicates that individuals who share more genetic material are more likely to show similar patterns of depressive symptoms.

These results demonstrate how genetic similarities are used in research: by comparing groups with different levels of shared DNA, researchers can infer the role of genes in behaviour.