To what extent does IQ 'explain' socio-economic variations in function?

Background: The aims of this study were to examine the extent to which higher intellectual abilities protect higher socio-economic groups from functional decline and to examine whether the contribution of intellectual abilities is independent of childhood deprivation and low birth weight and other socio-economic and developmental factors in early life. Methods: The Maastricht Aging Study (MAAS) is a prospective cohort study based upon participants in a registration network of general practices in The Netherlands. Information was available on 1211 men and women, 24 - 81 years old, who were without cognitive impairment at baseline ( 1993 - 1995), who ever had a paid job, and who participated in the six-year follow-up. Main outcomes were longitudinal decline in important components of quality of life and successful aging, i.e., self-reported physical, affective, and cognitive functioning. Results: Persons with a low occupational level at baseline showed more functional decline than persons with a high occupational level. Socio-economic and developmental factors from early life hardly contributed to the adult socio-economic differences in functional decline. Intellectual abilities, however, took into account more than one third of the association between adult socioeconomic status and functional decline. The contribution of the intellectual abilities was independent of the early life factors. Conclusion: Rather than developmental and socio-economic characteristics of early life, the findings substantiate the importance of intellectual abilities for functional decline and their contribution - as potential, but neglected confounders - to socio-economic differences in functioning, successful aging, and quality of life. The higher intellectual abilities in the higher socioeconomic status groups may also underlie the higher prevalences of mastery, self-efficacy and efficient coping styles in these groups.