Depressive symptoms in the Belgian population: disentangling age and cohort effects

Although the association between age and depression has been previously demonstrated, uncertainty remains because of the confounding relationship existing between age and cohort. A study by Yang (J Health Soc Behav 48(1):16, 2007) has evidenced important cohort effects and age-by-cohort interactions in depressive symptoms among US citizens. A crucial limitation, however, is that this study confines itself to elderly population. The objective of the present study is to bring further clarification to the association between age, cohort membership and depressive symptoms, by analyzing a sample with a wider age range.

The Panel Study of Belgian Households is a prospective longitudinal survey, following adults ages 25-74, annually from 1992 to 2002. Missing data were replaced using multiple imputation, allowing for a complete dataset (N = 7,000) at each wave. Respondents were classified into one of five birth cohorts: 1918-1927; 1928-1937; 1938-1947; 1948-1957; 1958-1967. Frequency of depressive symptoms was reported using a modified version of the Health and Daily Living form. Growth curve modeling was used to determine the effect of age and cohort on depression trajectory.

All cohorts differed significantly from one another, with recent cohorts always obtaining the highest mean HDL-depression score. The intensity of depressive symptoms increases linearly with age, but significant age-by-cohorts interactions were detected, indicating that the relationship between age and depression varies across cohorts. No evidence of a WW2 effect was found.

The association between age and depression has to take cohort membership into account. Cohort replacement effects explain the increase in depression in Belgium.