Contribution of IQ in young adulthood to the associations of education and occupation with cognitive ability in older age.

Background: Studies suggest that a higher education and occupation are each associated with a higher late-life cognitive ability, but their inter-relationships in their association with cognitive ability and the contribution of peak IQ in young adulthood ('pre-morbid IQ') often remain unclear.

Methods: Cross-sectional analysis of 623 participants aged ≥65 years of the BioCog study. Education was coded according to the International Standard Classification of Education (ISCED; range 1 to 6). Occupation was coded as 'semi/unskilled', 'skilled manual', 'skilled non-manual', 'managerial', 'professional'. A summary score of global ability ('g') was constructed from six cognitive tests. Pre-morbid IQ was estimated from vocabulary. The Geriatric Depression Scale assessed symptoms of depression. Age- and sex-adjusted analyses of covariance were performed.

Results: Education (partial eta 0.076; p < 0.001) and occupation (partial eta = 0.037; p < 0.001) were each significantly associated with g. For education, the association was attenuated but remained statistically significant when pre-morbid IQ was controlled for (partial eta 0.036; p < 0.001) and was unchanged with additional adjustment for depression (partial eta 0.037; p < 0.001). For occupation, the association with g was no longer significant when pre-morbid IQ (partial eta = 0.015; p = 0.06) and depression (partial eta = 0.011; p = 0.18) were entered as covariates in separate steps. When education and occupation were entered concurrently into the fully adjusted model, only education was independently associated with g (partial eta 0.030; p < 0.001; occupation, p = 0.93).

Conclusion: While a higher education and a higher occupation were each associated with a higher late-life cognitive ability, only for education some unique contribution to cognitive ability remained over and above its relationship with pre-morbid IQ, depression, and occupation. Further research is needed to address whether a longer time spent in education may promote late-life cognitive ability.