The preceding three sections reviewed studies on midlife or late life cognition with childhood SEP, education and adult SEP as exposures of interest, irrespective of whether they adopted the lifecourse perspective. In this section life course studies of SEP and mid to late life
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cognitive function are reviewed. These studies included SEP measures from childhood, adulthood and midlife or old age. Education was typically distinguished from other adult SEP measures and interpreted as a measure of own SEP in young adulthood. A complete summary of life course studies of SEP and cognition is shown in Table 1.1.
In all cross-sectional studies (122,123,172,173) but one (13) SEP measures from all three stages of the life course were independently associated with cognitive function in mid or later life. In these studies associations between childhood SEP and cognition were notably attenuated in the presence of education and adult SEP. The attenuated associations generally remained statistically significant but were mostly of modest strength. In all these studies (122,172,173) the associations between SEP measures from across the life course and cognition were positive and typically graded. In addition, associations of summary scores based on the number of life course stages spent in low/high SEP were also positive and graded, and were interpreted as evidence of a cumulative effect of SEP across the life course on cognition later in life. In contrast, in the Whitehall II study (13) the associations of childhood SEP and education with midlife cognition were fully mediated by adult SEP.
However, this study was based on data from an occupational cohort of British civil servants and may not be representative of the general population.
The pattern of life course associations observed in longitudinal studies broadly agrees with the findings from cross-sectional studies but with an important difference of introducing controls for prior cognitive ability. Notably, Richards and Sacker’s (126) path analysis of the 1946 British birth cohort showed a strong path from education and a weaker path from adult occupation to midlife cognition, whereas the direct path from father’s occupation to cognition was not substantively significant. The association between father’s education and midlife
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cognition was partially mediated by childhood cognitive ability, and, additionally, by education and occupation. Importantly, in this study the structure of associations was similar for measures of crystallized and fluid ability but the magnitude of the associations was greater for crystallized ability. Similarly, in the 1953 cohort of Danish conscripts (127) father's occupation, education and adult SEP were independently associated with age 57 cognition in ascending order of strength, controlling for adolescent cognitive ability. In contrast, only education was independently associated with age 70 cognitive ability in the 1921 Scottish birth cohort, controlling for childhood cognitive ability (128). Childhood cognitive ability and education also fully mediated the effects of father’s occupation and childhood material deprivation on age 70 cognitive ability. Current SEP was not associated with age 70 cognitive ability. However, the measure of current SEP was an index of neighbourhood quality, which may be a less precise indicator of individual-level SEP.
In addition, intergenerational social mobility (upward/downward mobility relative to parental SEP) was examined in three studies (122,172,173). Upwardly mobile individuals had better cognitive function than stable low SEP individuals but not as good as stable high SEP individuals, and vice versa for downwardly mobile individuals. One study (147) examined intra-generational social mobility by considering upward/downward change in own SEP between ages 26 and 53 in the 1946 British birth cohort. This study found a graded positive association between social mobility and midlife cognitive function, adjusting for education and initial cognitive ability. This suggests that the life course social trajectory may be modified by adverse or favourable socioeconomic changes in adulthood.
Overall, life course studies suggest that the association between childhood SEP and cognitive function in mid and later life is mediated by cognitive development, education and adult SEP.
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In addition, each of these mediators appears to be independently associated with cognitive function in descending order of strength.
However, several studies on life course SEP and cognition in midlife or older age suggest that the associations between SEP and cognition may be modified by contextual factors. Richards et al. (174) found the structure of life course associations between SEP and adult literacy and numeracy to be relatively stable across two UK birth cohorts but the changing strength of these associations varied in accordance with social structural changes, such as educational expansion, that occurred in the post-war period. Of the three birth cohorts studied by Clouston et al. (144) those with lower adolescent cognition benefited more from education in the U.S. cohort than in the British cohorts, while in the 1946 British cohort women derived a greater cognitive benefit from university education than men but were less likely to attain it.
The authors suggested that cognitive and socioeconomic returns to education may be affected by similar forces. Finally, a study of Chinese individuals (175) reported a similar role of social context in shaping the association between childhood SEP and cognition across successive cohorts. These studies suggest that at all stages of the life course the associations between SEP and cognition may be modified by contextual factors.
Most of the aforementioned life course studies come from Western populations. Among studies in non-western settings, two studies in China (175,176) and one study in Latin America (177) showed an association between life course disadvantage and greater odds of cognitive impairment in older age. Thus far no study in non-western settings examined the associations in normal cognitive ageing or included multiple cognitive tests spanning various cognitive domains. Moreover, no studies using data from Central and Eastern European populations were identified.
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Table 1.1. Characteristics of included studies on childhood and life course SEP and cognition Author Study
design Year Population Exposure measurement
Outcome
measurement Covariates Methods Findings
Kaplan cognitive ability at ages 8, 15 and 26 and cognition at 43 and 53 years. Long-term effects of early adversity explained by earlier cognitive ability and adult SEP, except for letter
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Based on the same cohort as Kaplan et al. (2001). Cumulative SEP scores
Path analysis Direct associations of age 15 IQ and education with cognitive measures.
Direct associations of high school mental activities with education and verbal fluency.
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and males, (n=349), mean age 74.8 years
longest-held education speed) No association of occupational
demands with cognition; explained
Strong association of age 11 IQ and weaker of education with age 70 IQ.
Father’s occupation and childhood deprivation associated with age 11 IQ, negative effect of father’s education. Neighbourhood quality not associated with age 70 IQ.
Haan
Path analysis Education strongly associated with cognition (but not with perceptual
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Historically, Central and Eastern Europe had smaller income inequalities than other countries at similar or higher levels of development and studies in Central and Eastern European populations may add a broader dimension to existing research on life course associations between SEP and cognition. In the second half of the 20th century Communist regimes tried to minimize material inequalities between different social groups, and this resulted in relatively weak correlations between education or occupation and income. This and other aspects of social stratification in Central and Eastern Europe under Communism were discussed in detail in Section 1.4. If the distinct socialist social stratification pattern can be assumed to have been consequential for life course accumulation of risk, then this may be reflected in the structure of life course associations between SEP and cognition of middle-aged and older individuals in these populations. Consequently, an investigation of life course associations between SEP and cognitive function in mid and later life in four Central and Eastern European populations constitutes one of major aims of this thesis.
In concluding this section, it is worth reiterating that beyond any direct effects of SEP on cognition, the plausible pathways linking SEP to mid-late life cognitive function include the underlying social gradient in health, which may be partly driven by differences in health-related behaviours. In Section 1.4. alcohol consumption and smoking have been identified as significant factors contributing to the high cardiovascular disease burden and premature mortality in the region. At the same time, both childhood and adult socioeconomic circumstances are important determinants of CVD risk (178), and, in turn, cardiovascular risk factors are likely to be important for cognitive ageing (179). Since alcohol consumption and smoking behaviour tend to be socially patterned, it is anticipated that in these four Central and Eastern European populations both alcohol consumption and smoking may be important mediators of the associations between life course SEP and mid-late life cognitive function as
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well as likely independent risk factors for cognitive function. The latter is the subject of the next section.