homicide may have a common explanation in the life experiences of the victims.

In short, existing research points to a variety of pathways that could link early childhood exposures and late adult conditions (e.g., diabetes, cardiovascular disease). The same pathways might also explain correlations between health status across contiguous stages of the life cycle, spanning shorter time intervals, such as between infancy and early childhood or between adolescence and young adulthood.

However, early life experiences are hardly the only influences on health and mortality later in life. It is important to differentiate between the health trajectories experienced by individuals and the epidemiologic trends observed in populations over time. For the latter, “period effects” can play a major role in explaining cross-national differences. For example, increases in smoking during the first half of the 20th century and in obesity in the second half were largely period effects that touched adults of all age groups and eventually extended to adolescents and children as well. Similarly, U.S. tobacco control efforts designed to reduce smoking among adults, which achieved success from the 1960s onward (see Chapter 5), benefited people of all ages.

The panel was acutely aware of one of the most well-known and vexing challenges when studying nonmedical influences on health, that of describing and empirically demonstrating causal pathways between a given health factor and a biological health outcome. In other words, how do the conditions presented by family, community, and national environments get under a person’s skin to affect health? And how do these conditions affect people differently? Even twins do not experience the same health development trajectories over time (Madsen et al., 2010). Unlike the study of clinical interventions or biological effects, research on social, environmental, and policy factors involves more multidisciplinary and varied methodological approaches, including different notions in the epistemology of what constitutes “evidence” (Anderson and McQueen, 2009; Braveman et al., 2011c; McQueen, 2009; Rychetnik et al., 2002; Victora et al., 2004).10 As the Measurement and Evidence Knowledge Network (Kelly et al., 2006, p. 33) of the WHO Commission on the Social Determinants of Health noted:

The data and evidence which relate to social determinants of health come from a variety of disciplinary backgrounds and methodological traditions. The evidence about the social determinants comprises a range of ways of knowing about the biological, psychological, social, economic and material worlds. The disciplinary differences arise because social history, economics, social policy, anthropology, politics, development studies,

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10The limitations of randomized controlled trials are discussed in Chapter 6.



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