In epidemiologie samples, the assessment of lifetime history (LTH) of major depression (MD) is not highly reliable. In female twins, we previously found that LTH of MD, as assessed at a single personal interview, was moderately heritable (approximately 40%). In that analysis, errors of measurement could not be discriminated from true environmental effects.
In 1721 female twins from a populationbased register, including both members of 742 pairs, LTH of MD, covering approximately the same time period, was obtained twice, once by self-administered questionnaire and once at personal interview.
Reliability of LTH of MD was modest (×= + .34, tetrachoric r= + .56) and was predicted by the number of depressive symptoms, treatment seeking, number of episodes, and degree of impairment. Deriving an "index of caseness" from these predictors, the estimated heritability of LTH of MD was greater for more restrictive definitions. Incorporating error of measurement into a structural equation model including both occasions of measurement, the estimated heritability of the liability to LTH of MD increased substantially (approximately 70%). More than half of what was considered environmental effects when LTH of MD was analyzed on the basis of one assessment appeared, when two assessments were used, to reflect measurement error.
Major depression, as assessed over the lifetime, may be a rather highly heritable disorder of moderate reliability rather than a moderately heritable disorder of high reliability.