This study of the association of childhood and adult social class with indicators of physical limitations (limitations in running 100 m and dental status), general health (self-rated health) and early frailty (fatigue) among randomly sampled middle-aged men and male twins revealed that low social class in both childhood and adulthood were associated with having poorer health outcomes. When mutually adjusted childhood social class explained a minor part of the relation between adult social class and these health outcomes. This suggests that social differences in poor health in middle-age are due to factors operating both in adult- and to a certain degree in early life. However, adult social disadvantage does not seem to determine poor dental status when early life factors are taken into account.
Physical functioning has also been associated with both childhood and adult social class in birth cohorts from UK (Guralnik, Butterworth, Wadsworth, & Kuh, 2006
; Kuh et al., 2006
). In contrast to our finding, the effect of childhood social class on health outcomes in the studies from UK was entirely explained by adjustment for adult behavioral factors such as smoking and BMI. However, all the studies suggest that factors related to adult social class mediate the effect of childhood social class. In addition, our study showed that male twins discordant for adult social class seem to differ in prevalence of limitations in running, which suggests that social differences in this measure of physical functioning in adulthood are also due to factors operating later in adult life. The strong relationships of childhood and adult social class with poor dental status are in agreement with a few previous studies (Antoft, Rambusch, Antoft, & Christensen, 1999
; Thompson et al., 2004
). In a birth cohort from New Zealand, social class and oral health during childhood predicted oral health at age 26 (Thompson et al., 2004
). In this study upward social mobility between age 6 and 26 years was also associated with a reduced risk of dental caries and tooth loss. This is in contrast with the findings in the present study where male twins discordant for adult social class (different social mobility) did not differ in prevalence of poor dental status. This suggests that the observation of an effect of adult social class on dental status in the individual level analyses (Tables and ) could be due to confounding from genetic factors or rearing environment (early selection mechanisms ()). In Denmark dental care is one of the few health benefits that are not free of charge. Consequently, we were surprised that in the co-twin design adult social class did not seem to have an independent effect on dental status. We cannot exclude the possibility of some effect of adult social class on dental health, however, as the twin-analyses had insufficient power to detect a modest difference. However, when the three discordant categories where collapsed in order to gain power, the effect estimate moved close towards unity.
The finding of a relationship of both childhood and adult social class with self-rated health in middle-age found in our individual level analyses, is in agreement with studies within other populations (Adams, White, Pearce, & Parker, 2004
; Lundberg, 1993
; Mensah & Hobcraft, 2008
; Van de Mheen, Stronks, Looman, & Machenbach, 1998
; Power, Matthews, & Manor, 1998
). Previous studies have shown that illness in childhood explains a part of social class differences in self-rated health around age 30 years (van de Mheen, Stronks, Looman, & Mackenbach, 1998
). Unfortunately, we had no information on participant’s health during childhood, but in the co-twin control study, which allows us to adjust for all mediators or confounders linked to genetic factors or rearing environment — most likely including propensity to childhood disease — we continued to observe social class differences in poor self-rated health. These differences are consistent with a causal effect of current social class exposures, although selection effects related to poor health during the life course cannot be excluded (later selection mechanisms ()). In previous studies higher education, social class and income have been associated with less fatigue (Avlund, Rantanen, & Schroll, 2007
; Watt et al., 2000
), but we are not aware of studies that have explored the effect of childhood experiences on this outcome.
In the present study we have included two independent study samples, which has the advantage that it allows us to make confirmatory analyses. This means that even though differences in social class and health between twins and singletons might potentially influence the generalizability of the findings based on the random sample of middle-aged Danish twins (MADT), the fact that effect estimates obtained in the two different study samples () were fairly similar, suggests that this is not a serious concern. The twin design used in this study has some additional advantages as it enables us to control for both genetic and early environmental factors per design.
Some other methodological issues should be considered. First, the design was cross sectional. This hampers the examination of the causal pathways between childhood social class, adult social class and adult health. Our assumption is that childhood and adult social class affects adult health and not vice versa, but reverse causation in shape of direct and indirect selection processes cannot be excluded. However, in the Metropolit study we also had prospectively collected information on father’s occupational class at the birth of participants and subject’s own social class at age 18 and 48 years. When we repeated the analyses with these data it did not change the pattern of associations found in the present cross sectional design. Secondly, all measures were self-reported and measurement errors in our assessment of social class, health outcomes and covariables might be a potential concern. If errors in reporting were non-differential this would bias the results towards the null. However, it is possible that poor health somehow has influenced the reporting of a lower social class, which may cause bias and explain the found associations. Thirdly, selection problems may be an issue since the present study was based on those who responded to a health survey. In the Metropolit study non-participation in the health survey in 2004 was associated with having a single mother at birth and low childhood cognitive function (Osler, Kriegbaum, Lund, Christensen, & Nybo Andersen, 2008a
; Osler, Kriegbaum, Holstein, Christensen, & Nybo Andersen, 2008b
; Osler et al., 2008a
). The incidence of hospitalizations for alcohol abuse, tobacco-related lung disease, and depression was also higher among non-responders. However, when analyzing the associations between information on early life factors and these health outcomes, which were available for all cohort members, it showed that the risk estimates did not differ significantly between responders and the entire cohort. Thus, non-response does not seem to bias the associations between childhood social class and adult health seriously. Finally, we need to emphasize the relatively small number of twin pairs, which affects the power of our data set to detect all differences for the least prevalent health outcomes such as poor dental status. On the other hand, this strengthens the findings on poor health and fatigue, although we cannot exclude the occurrence of change findings as a result of the multiple comparisons made.
In conclusion, the present study suggests that in Denmark the associations of adult social class with physical limitations, poor self-rated health and fatigue may partly be due to effects of exposures related to social circumstances during adult life, while social class differences in dental status are most consistent with early effects.