Commentary: influence of early life intelligence test performance on later health: do lower scoring children become less healthy adults?

the introduction of intelligence testing in the early 20th century, this characteristic (also known as cognition, cognitive function, and mental ability) has consistently been shown to be related to indices of socioeconomic position, such as education and occupational social class, with the least favourable levels seen in individuals from the poorer backgrounds. 1 Childhood cognitive function has also been suggested as one of the mechanisms mediating the association between socioeconomic position in early life and adult mortality. Cognition measured in early life has been related to final qualifications 1 and adult behaviour, such as smoking, 2 supporting the notion that this psychometric characteristic may be associated with health. However, given the ubiquitous social patterning of cognition scores, a potential alternative explanation for this effect is confounding by social circumstances. Further indirect support for this view point can be found in the patterns of association between childhood cognition and adult cause-specific mortality which tend to reflect those reported for paternal social class, a widely used indicator of pre-adult socioeconomic position. Although pre-adult cognition seems to relate to adult disease aetiology—as mediator and/or a predictor in it own right—it is only in within the last decade that intelligence has been examined in relation to health outcomes. The reticence of investigators to fully grasp this area of enquiry may be ascribed to its contentious nature, as exemplified in the vigorous debates that followed the release of Herrnstein and Murray's The Bell Curve: Intelligence and Class Structure in American Life, 3 one of the most provocative social science publications of recent years. In this text, analyses of data from the US National Longitudinal Survey of Youth were used by the authors to illustrate ethnic differences in general intelligence, leading to their assertion that eugenic-like reforms should be made to the modern US welfare system. Notably, their findings were never subjected to the scrutiny of peer review. Another explanation for the reticence may be due to the questions that have been raised as to what IQ tests actually measure. Until recently, the literature on childhood cognition and health has been dominated by non-representative samples of army recruits 4 and nuns, 5 not all of which provided direct measures of cognition—in the latter cohort, for instance, mental ability measurements were based on the density of information in selected passages from convent members' handwritten autobiographies. The paucity of data from populations which represent the full range of …