Trajectories of depression with age

I will discuss recent work where we aimed to estimate age trajectories of depression by combining multiple samples. The analysis sample included 16,881 twins aged 25-102 years drawn from the Interplay of Genes and Environment across Multiple Studies (IGEMS) consortium representing 9 studies from three countries (USA, Denmark, and Sweden). Growth curve analyses results indicated, however, that the trajectories differed considerably by country, and country covariates captured this very differently from country-specific analyses. In the multi-country analysis, greater age was associated with lower depression scores at intake, and countries had contrasting depression trajectories. For Sweden, depression scores decreased at the youngest ages, before levelling out and increasing in the oldest age groups. However, for Denmark, depression scores inclined more steeply with greater intake age. In the Sweden-only analysis, younger age was associated with higher depression scores at intake. In the Danish-only analysis, greater age at intake was associated with higher depression scores, and with greater intake age the trajectory gradually became more concave. Females had higher depression scores at intake in both the Danish and Swedish samples. The USA data were too limited to form any consistent trajectories of depression. As it remains unclear whether multi- or individual-country analysis more accurately reflected the data, our next step is to fit age-based models. Although a weaker longitudinal model, they may offer a contrasting perspective that can help resolve this question.