A gene is a hereditary unit within DNA that encodes specific traits or functions. All nucleated cells in the human body contain chromosomes, which are thread-like structures holding DNA molecules. DNA stores information as a sequence of four chemical bases, paired to form a double helix structure. This sequence is divided into 23 chromosomes, with each pair inherited from one's parents.

Psychologists use genetic relationships to hypothesise the influence of genetics and environment on behavior, with higher genetic similarity expected to correlate with greater behavioural similarity. Twin research is a key method in studying genetic inheritance and behavior, as twins share common genetic material. Monozygotic (MZ) twins are genetically identical, originating from one fertilised egg, while dizygotic (DZ) twins come from two eggs and share about 50% of their genes, similar to regular siblings. MZ twins generally show higher concordance rates for inherited traits compared to DZ twins.  A concordance rate is an extent to which a given trait is shared by a pair of individuals expressed as a percentage. Heritability is calculated using concordance rates, as the rates inform the variance of a given trait, and seeks to express the variance.

Twin research used to operate under the assumption that any major difference in behavior, disease or physiological trait that appeared in one twin and not the other was a result of environmental factors, and therefore it was assigned a low heritability. This is because if the trait was a result of genetic factors, both twins would manifest the trait.  However, knowledge of epigenetics has changed this assumption and has therefore created a more complex and nuanced approach to monozygotic twin research.

Kendler et al. (2006) conducted a study to determine the role of genetics in major depressive disorder. They aimed to verify past findings on heritability, examine gender differences, and explore whether genetic and environmental factors change over time. The study involved 15,493 complete twin pairs from the Swedish Twin Registry. Trained interviewers conducted telephone interviews between March 1998 and January 2003, using modified DSM-IV criteria to assess lifetime major depression. The interviews also gathered information on shared and individual-specific environments.

The results showed higher concordance rates for major depression in monozygotic (MZ) twins compared to dizygotic (DZ) twins, with female MZ twins having a concordance rate of 0.44 and male MZ twins 0.31, while female DZ twins had a concordance rate of 0.16 and male DZ twins 0.11. The overall heritability of major depression was estimated at 0.38, consistent with previous research. The study found that genetic factors played a more significant role in women than in men and identified no significant differences in genetic and environmental influences across different birth cohorts or between pre- and post-World War II cohorts.

This study underscores that heritability of major depression is higher in women and that some genetic risk factors are sex-specific, confirming the reliability of European twin studies in estimating the heritability of major depression.