Kendler et al. (2006) - Genetic similarity

Question 1

Aim

Answer 1

To investigate whether major depressive disorder may be inherited. They also wanted to see if there were significant gender differences in the heritability of major depression.

Question 2

Sample

Answer 2

The sample was made up of 15,493 complete twin pairs.

Question 3

Method

Answer 3

In order to gather their data, the researchers used a team of trained interviewers to carry out telephone interviews. 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.

In addition to this information, the interviewers also asked questions about the twins’ “shared environment.”

The researchers found a 44% concordance rate for MZ female twins and 31% for MZ male twins – compared to a 16% concordance rate for DZ female twins and 11% for DZ male twins.

Question 4

Results

Answer 4

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

Question 5

Evaluation

Answer 5

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.