Young adult cognitive ability and subsequent major depression in a cohort of 666,804 Danish men.

Background: Early life cognitive ability (CA) might influence the risk of developing major depression (MD). The aim was to investigate the association between young adult CA and subsequent MD in relation to different MD disease characteristics.

Methods: Information on CA was assessed at conscription board examinations 1957-1984 (mean age 19 years) and information on MD was based on hospital diagnosis retrieved from Danish Patient registers 1969-2015. Associations between CA and MD were examined using Cox regression analyses.

Results: A total of 666,804 men (born 1939-1959) were followed and 25,841 (3.9%) developed MD during a mean follow-up of 40.8 years. Lower CA was associated with an increased risk of incident MD. The association was stronger for early-onset (<60 years) (HR = 1.23; 95%CI:1.21,1.24) compared to late-onset (≥60 years) MD (HR = 1.14; 95%CI:1.11,1.16), but CA was not related to number of depressive episodes. The association was stronger for single depressive episodes (HR = 1.21; 95%CI:1.19,1.23) compared to recurrent depression (HR = 1.13; 95%CI:1.09,1.16), while the strength of the association did not differ according to MD disease severity (ICD10: mild, moderate, and severe depression).

Limitations: The study sample only included men and only MD cases diagnosed at hospital were included which limits the generalizability.

Conclusion: Low CA could be a risk factor for especially early onset MD in men, whereas the influence of CA on re-occurrence seems less strong. Lower pre-morbid CA increases the risk of MD and should therefore be part of the depression risk assessment in clinical practice.