Kendler and his team wanted to investigate three questions in their study:

1. Past studies suggest a 35 - 45% heritability of major depression. Would this be true in a large Swedish sample?
2. Are there significant gender differences in the heritability of major depression?
3. Is there evidence that genetic and environmental factors in major depression differ over time?

The sample was made up of 15,493 complete twin pairs listed in the national Swedish Twin Registry. The twins were born between 1886 and 1958. Only twins whose zygosity could be verified were used in the study.

In order to gather their data, the researchers used a team of trained interviewers to carry out telephone interviews.  Interviews were carried out between March 1998 and January 2003. The interviewers assessed lifetime major depression by using modified DSM-IV criteria. 8056 twins met the criteria for a diagnosis of major depression at some point in their life - and 322 twins voluntarily discussed a history of antidepressant treatment.

In addition to this information, the interviewers also asked questions about the twins "shared environment" - that is, when they were living in the same household - and their "individual-specific environment" - that is, adult personal life events that may make members of the twin pair more susceptible to depression.

Correlation of Lifetime Major Depression and Zygosity

The results indicate that the concordance rates for major depression were significantly higher in women than men. In addition, the correlations were significantly higher in monozygotic than in dizygotic twins. They also found no correlation between the number of years that the twins had lived together and lifetime major depression. The estimated heritability of major depression was 0.38, in line with previous research.

There were also no significant differences seen in the roles of genetic and environmental factors in major depression in the three cohorts spanning birth years 1900-1958. Even when they split the entire cohort into pre and post World War II, there was no significant difference.

This study suggests both that the heritability of major depression is higher in women than in men and that some genetic risk factors for major depression are sex-specific. In addition, the study confirms the level of heritability of major depression found in other studies, strengthening the reliability of European twin studies.

• The study is correlational, so no cause and effect relationship can be determined. No particular genes were isolated and tested in the study.
• The study appears to confirm previous research, strengthening the reliability of the findings.
• Information about life-events and depressive symptoms was self-reported.  Differences could arise if men are less reliable in their reporting of lifetime major depression than women.
• The interviewers did not officially diagnose the twins and they accepted the diagnoses made by clinicians when it was reported by the interviewee; as we know, clinical diagnoses are not highly reliable and making such a diagnosis by telephone may be considered of questionable validity.
• The very large sample size taken from a single population helps to mediate some of the concerns above.