Can the influence of childhood socioeconomic status on men's and women's adult body mass be explained by adult socioeconomic status or personality? Findings from a national sample.

Obesity is a major public health problem in industrialized societies (Cope & Allison, 2006; McLaren, 2007), resulting in levels of morbidity nearly comparable to smoking and poverty (Sturm & Wells, 2001). Although the direction and strength of social gradients in obesity may vary depending a country's level of industrialization and age cohort, in many countries, social inequalities in body mass appear relatively early in life (McLaren, 2007; van der Horst et al., 2007) and endure through adulthood. Importantly, individuals born into higher childhood socioeconomic status (SES) show lower rates of adult obesity (Sarlio-Lahteenkorva, 2007). A major question, however, is whether associations between lower childhood SES and adult obesity can be explained by adult SES (Drewnowski & Specter, 2004; Klohe-Lehman et al., 2006). If the effects of childhood SES on adult weight status are mediated entirely through adult SES, then prevention and intervention resources altering the path from childhood social disadvantage to adult social disadvantage may have a substantial impact on reducing weight-related morbidity in adulthood. If, on the other hand, childhood SES effects on adult weight status can be only partially explained by adult SES, then other paths from social early social disadvantage to adult weight problems exist, requiring additional consideration in prevention and intervention efforts. Some reports indicate that adult SES completely mediates associations between childhood SES and adult obesity (Lawlor et al., 2005; Regidor, Gutierrez-Fisac, Banegas, Lopez-Garcia, & Rodriguez-Artalejo, 2004), while others indicate substantial residual effects for childhood SES after adult SES has been controlled (Laaksonen, Sarlio-Lahteenkorva, & Lahelma, 2004; Langenberg, Hardy, Kuh, Brunner, & Wadsworth, 2003). However, associations between obesity and both adult SES (Zhang & Wang, 2004) and childhood SES appear stronger in women (Case & Menendez, 2007; Laaksonen et al., 2004). This raises the possibility that potential residual childhood SES risk for unhealthy adult weight is gender-specific. However, weight status in adulthood is a function not only of social gradients in the distribution of wealth and resources, but also of individual differences in behavioral tendencies and psychological factors (Cope & Allison, 2006; Glass & McAtee, 2006). A second question therefore is whether the associations of childhood social class and adult weight status can also be explained to some extent by individual personality characteristics in adulthood. The “Big Five” phenotypic personality traits of Neuroticism, Extraversion, Openness to Experience, Agreeableness, and Conscientiousness comprise the major axes of dispositional behavioral variation in humans (Goldberg, 1993), and constitute “intermediate phenotypes” reflecting a joint expression of both genetic and social environmental factors influencing health (Institute of Medicine, 2006). Three traits have been implicated directly in weight gain in adulthood (Brummett et al., 2006): lower Extraversion (or low levels of positive mood, energy, and sociability) in men, higher Neuroticism (or high levels of or stress reactivity and general negative mood) in women, and lower Conscientiousness (or low levels of or reliability, achievement striving, and self-discipline) in both genders. How do personality and socioeconomic forces work in conjunction to influence adult weight status? Life course epidemiology (Ben-Shlomo & Kuh, 2002; Kuh, Ben-Shlomo, Lynch, Hallqvist, & Power, 2003) provides at least three conceptual models. First, a risk chain model with triggering effects suggests that lower childhood SES influences adult weight status entirely through lower adult SES, and/or personality traits. In other words, the initial risk of early social disadvantage triggers later risks, such as lower adult SES and low Conscientiousness, that are proximally tied to obesity. Some studies provide support for such a model, documenting complete mediation of childhood SES by adult SES (Lawlor et al., 2005; Regidor et al., 2004). However, when substantial residual associations between childhood SES adult obesity are observed after adjusting for adult SES (Laaksonen et al., 2004; Langenberg et al., 2003), the risks of childhood SES work through additional pathways, suggesting a second risk chain model of partial mediation. A third risk accumulation model with independent additive effects suggests that the association of early exposures, such as childhood social disadvantage, with adult obesity is essentially unmediated by adult personality. The latter may independently add to (exacerbate) and/or subtract from (compensate for) the risk conferred by childhood and adult SES. If such a model prevails, the risk conferred by disadvantaged childhood SES may be offset by protective traits in adulthood such as high Conscientiousness, or the benefit of childhood advantage may be erased by personality traits linked to obesity and higher BMI. In this case, adult personality traits would be useful for explaining heterogeneity in weight status among individuals of similar childhood or adult SES, constituting an important “individual” level adjunct to societal factors, such as socioeconomic gradients, which influence weight status (Cope & Allison, 2006). Virtually no studies have reported on the extent to which childhood social disadvantage affects standing on adult personality traits. However, a model of independent additivity for personality and SES would appear consistent with a recent report that adult temperament and childhood SES exerted largely independent effects on adult waist-circumference (Sovio, King, Miettunen, Ek, Laitinen, Joukamaa, et al., 2007). Other findings have documented independent associations between adult BMI and childhood temperament, independent of childhood SES (Pulkki-Raback, Elovainio, Kivimaki, Raitakari, & Keltikangas-Jarvinen, 2005) and BMI and childhood Big Five traits, independent of education (Hampson, Goldberg, Vogt, & Dubanoski, 2006). To the extent that childhood personality is associated with adult personality, these studies provide further clues about the possible independent additivity of adult personality. We examined the joint influences of childhood SES, adult SES, and adult personality traits on obesity and BMI in a United States cohort with rich data on both the Big Five and SES in childhood and adulthood. Based on previous findings (Laaksonen et al., 2004; Zhang & Wang, 2004), we hypothesized that residual effects of childhood SES on adult weight status would be observed for women, but not men. This corresponds to different life course epidemiologic risk chain models in which childhood SES effects are partially mediated by adult SES in women, but completely mediated or triggered by adult SES in men. We further hypothesized that adult personality would be associated with weight status independently of SES, explaining additional variation in BMI and obesity (Sovio et al., 2007). Specifically, we expected low Extraversion in men, high Neuroticism in women, and low Conscientiousness in both genders to be associated with higher adult body mass (Brummett et al., 2006). From the perspective of life-span epidemiologic models, we thus expected independent additive risks for adult personality vis-a-vis SES in both genders, though we suspected that the particular traits associated with weight status would differ in men and women.