At the outset of this investigation, we identified three main research questions. First, does birth cohort membership influence the odds of obesity in the U.S.? Second, are period effects the predominant force behind the obesity epidemic? Third, do age, period and cohort effects differ across groups defined by race/gender and educational attainment? Results of CCREMs for 1.7 million NHIS participants over the period 1976-2002 answered each of these questions with an unambiguous “Yes.”
The answer to the first of these questions is most vital to our study—and it is worth reiterating. Independent of age and period effects, birth cohort membership has significantly influenced the odds of obesity in the U.S. Relative to cohorts born toward the end of the nineteenth century, cohorts born during the first half of the twentieth century tended to experience declining odds of obesity. However, the predicted probabilities of obesity for more recent birth cohorts have tended to increase since the late 1950s and early 1960s. This indicates that newer birth cohorts have suffered a form of double jeopardy in which ubiquitous secular changes and birth cohort membership have independently contributed to increased odds of obesity. In the context of secular changes that have caused obesity rates to increase sharply across nearly all social and demographic groups in the U.S., the trends we observed among recent birth cohorts are deeply troubling. As noted recently in a series of life table analyses, life expectancy in the U.S. could decline in the near future if obesity trends persist (Olshansky, Passaro, Hershow, Layden, Carnes, & Brody et al., 2005
). Although this rather dire prediction has been the subject of much criticism, our finding that newer birth cohorts are increasingly susceptible to obesity—even as period trends continue upward—suggests that there is some cause to heed Olshanksy's warning. Furthermore, because our findings were broadly consistent with those reported by Allman-Farinelli et al. (2008)
for the Australian population, we have some concern that the health and longevity of populations in other developed nations could also be in jeopardy.
The cohort trends we observed are consistent with assertions that newer birth cohorts permit social change through rapid cultural and technological adaptation (Ryder, 1965
). Although the mechanisms that have caused obesity to increase among more recent birth cohorts are not well understood, one possibility is that the amount of time that children and adolescents spend in front of computers and video games has increased rapidly in recent years (Subrahmanyam, Kraut, Greenfield, & Gross, 2000
). While some studies suggest that computers and video games do not substantially contribute to sedentary behaviors among children and adolescents (e.g., Sturm, 2005
), others indicate that these technologies do not substitute for television viewing but rather increase the total amount of “screen time” (Coffee & Stipp, 1997
; Nielsen Media Research, 1999
). Computer use is also positively associated with the consumption of soft drinks and snacks among middle- and high-school students, perhaps due to advertisements that target youths through internet media (Utter, Neumark-Sztainer, Jeffery, & Story, 2003
). Other plausible mechanisms include changing dietary patterns, sleep debt, pharmaceutical drug use (e.g., the rising use of antidepressants and antihistamines), and epigenetic factors, to name just a few (Keith et al., 2006
). The latter is an alarming possibility, as it suggests that recent birth cohorts may be predisposed to gain weight—and that they could pass on this predisposition to their own children. Given the potential threat to public health caused by the rising odds of obesity among newer birth cohorts, it is vitally important to begin the process of identifying these mechanisms—and determining their relative importance—so that appropriate interventions can be implemented.
Recent cohorts could also manifest higher rates of obesity than their predecessors through improved survivorship among obese persons. Analyses of NHANES data have found that, relative to persons of normal weight, the risk of mortality among obese persons has declined in the U.S. since the 1970s (Flegal, Graubard, Williamson, & Gail, 2005
). Reductions in mortality among obese persons are likely due to a combination of improving cardiovascular risk profiles among the obese (Gregg, Cheng, Cadwell, Imperatore, Williams, & Flegal et al., 2005
) and declining rates of coronary heart disease mortality (Gu, Cowie, & Harris, 1999
), which is the chief cause of death among obese persons in the U.S. (Flegal et al., 2005
). Improving mortality profiles among obese persons may seem at odds with bleak predictions about the future health and longevity of the U.S. population and, indeed, there are some who argue that medical progress could more than compensate for increasing obesity rates (e.g., Preston 2005
). However, the promise of continued medical progress should be balanced against compelling evidence that obesity at young ages can have serious long-term health consequences (see, for example, Freedman, Patel, Srinivasan, Chen, Tang, & Bond et al., 2008
). In any event, through the twin mechanisms of increasing probabilities of obesity and declining rates of cardiovascular mortality, new cohorts may contribute to the obesity epidemic through “demographic metabolism” in which older, leaner cohorts are replaced with newer, heavier cohorts (Ryder, 1965
, p. 843).
In addition to confirming the existence of birth cohort effects, our study affirms that period effects were principally responsible for the obesity epidemic. Independent of age andcohort effects, the predicted probability of obesity increased substantially over the period 1976-2002. Predicted probabilities of obesity increased for all groups studied, confirming the ubiquitous reach of the obesity epidemic. Secular changes responsible for strong period effects may include an increasing reliance on technology for work (French et al., 2001
; Lakdawalla & Philipson, 2002
), food preparation (Cutler et al., 2003
) and leisure activities (French et al., 2001
). Also, technological innovations have reduced food costs and nearly removed economic incentives to engage in physical activity (Lakdawalla & Philipson, 2002
; Philipson & Posner, 2003
). Moreover, modern technologies have been used to develop and transmit aggressive marketing campaigns designed to increase the consumption of high calorie foods (Brownell, 2002
; French et al., 2001
). While there is no shortage of speculation about the underlying causes of period effects, research has yet to disentangle the relative importance of these factors.
Results from this study support previous research showing that body mass increases rapidly through young adulthood but subsequently increases at a slower pace (Lewis et al., 2000
; Rissanen, Heliovaara, & Aromaa, 1988
). Since physical activity tends to decline during young adulthood (Sternfeld, Sidney, Jacobs Jr., Sadler, Haskell, & Schreiner, 1999
) and dietary behaviors may include frequent fast food consumption (French, Harnack, & Jeffery, 2000
), it is not surprising to observe increasing odds of overweight and obesity among young adults. The plateau observed during middle age may result from more careful monitoring of calorie intake (Lewis et al., 2000
), since available evidence suggests that basal energy expenditure and physical activity continue to decline at these ages (Crespo, Keteyian, Heath, & Sempos, 1996
; Starling, 2001
). These results are also consistent with findings that weight loss is common at older ages (Rissanen et al., 1988
). Ironically, this is not encouraging since weight loss in later life may result from muscular atrophy and is associated with increased risk of illness, hospitalization and mortality (Dey, Rothenberg, Sundh, Bosaeus, & Steen, 2001
; Roubenoff, 1999
Although age effects are parabolic for each demographic group that we studied, considerable differences in the predicted probabilities of obesity were nevertheless detected. For instance, the predicted probability of obesity increased and then declined more rapidly with age among Black women relative to other race/gender groups. We made a similar, though less pronounced finding for NHIS respondents with less than a high school education. Consistent with existing evidence, these findings indicate that Black women and persons without a high school diploma are at particularly high risk of developing obesity at some point during the life course. The sharp decline in obesity among undereducated persons and Black women as they aged may also indicate that individuals in these groups are particularly likely to suffer the adverse consequences of obesity, such as weight loss due to illness and premature mortality.
We also detected important differences in period and cohort effects for various demographic groups. For instance, while Black women did not exhibit substantially different period trends than other race/gender groups, their predicted probabilities of obesity were substantially higher than other race/gender groups across the entire period of observation. The predicted probabilities of obesity for cohorts of Black females were also consistently elevated. Also troubling was the very sharp increase in cohort effects that we observed for Black females who were born after 1955. Although the mechanisms responsible for this increase are not clear, they are broadly consistent with a fundamental social cause interpretation (Link & Phelan, 1996
). That is, relative to more advantaged demographic groups, young Black females may not be well equipped to mitigate the risks introduced by recent technological changes in U.S. society.
Another important finding in our study was the convergence of period trends for all groups with less than a college education. From 1976 through the mid-1990s, the predicted probabilities of obesity declined at each step up the educational ladder. But from the late 1990s onward, only a college education appeared to confer any protection against obesity. This finding may reflect improved high school graduation rates and increasing exposure to college over this period of observation (Newburger & Curry, 1999
). Persons without a college degree are increasingly disadvantaged in U.S. society, and part of this disadvantage appears to be reflected in the period trends that we observed. Race/gender and educational differences in age, period and cohort effects pose a significant challenge to laudable public health goals to reduce health disparities, such as those articulated in Healthy People 2010
(U.S. Department of Health and Human Services, 2000
Strengths of our study include a large nationally representative sample, micro-level data and refined measures of age, period and cohort. Also, the initial period of observation (1976) in our study preceded the sharp increase in obesity rates (Flegal et al., 1998
), suggesting that estimates for period effects were not attenuated by the inability to include earlier time periods. In addition, our investigation utilized a state-of-the-art methodological approach (CCREM) to help overcome problems that have historically plagued age-period-cohort analyses, such as collinearity and biased standard errors. This methodological approach enabled us to demonstrate that secular changes are principally responsible for the obesity epidemic, while simultaneously showing that the odds of obesity have increased for recent birth cohorts. The adoption of CCREMs also provides a framework for future analyses to incorporate covariates (e.g., food prices) that may account for rising rates of obesity. Presuming that the inclusion of such variables leads to substantial attenuation in period and/or cohort effects, it will contribute much to our understanding of the etiological factors responsible for the obesity epidemic.
The unavailability of direct, anthropometric measures of height and weight is a limitation of NHIS data, particularly since NHIS used a peculiar mix of proxy, partial-self and self-reports between 1976 and 1996. However, a unique quality of our investigation is that it accounted for reporting status and other biases through the development of a new adjustment procedure that caused BMI estimates in NHIS to approximate those based on NHANES examination data. Another limitation of this study is its inability to identify Hispanics. Between 1970 and 2000, the Hispanic population increased from 4.7 to 12.5 percent of the total U.S. population (U.S. Census Bureau, 2008
). Although obesity rates do not differ for Hispanic and non-Hispanic White men, the prevalence of obesity tends to be higher among Hispanic women than non-Hispanic White women (Ogden et al., 2002
). As a consequence, the period and cohort effects that we observed for non-Black females may be due in part to the changing ethnic composition of this group. Although similarities in our estimates for non-Black females and non-Black males suggests that the influence of Hispanics is relatively small, it is worth noting that the divergence in cohort effects for these two groups after 1965 may reflect the increasing influence of Hispanics on recent cohorts of non-Black females. Finally, this study is unable to assess whether the odds of obesity have continued to increase among cohorts born after 1984. Clearly, it is imperative to bring data to bear on this important question.
Future research should investigate obesity trends among more recent birth cohorts by conducting APC investigations of adolescents and young adults with existing resources (e.g., the National Longitudinal Study of Adolescent Health) and, if necessary, primary data collection. Emerging public health issues among children and adolescents such as type 2 diabetes and fatty liver—health problems that used to be true medical oddities in this age range—strongly suggest that the birth cohort trends we observed in this study have persisted for more recent birth cohorts (Ludwig, 2007
). If additional research confirms our findings, it could prove useful to develop cohort-specific strategies that limit weight increases early in the life course. Two possible strategies include targeted mass media campaigns (Bauman, 2004
) and the removal of obstacles to routine physical activity that did not exist for older birth cohorts (Dellinger, 2002
). As U.S. society continues to evolve, it may also be advisable to craft interventions that are sensitive to the formative social and cultural milieu that influence the values and preferences of individual birth cohorts, in much the same way that some health-related policies are sensitive to issues involving gender or race/ethnicity. Developing successful cohort-specific interventions is likely to be challenging, in part because the social changes responsible for the obesity epidemic are unlikely to reverse course, and also because the causes of and solutions to this problem are likely to be complex, multifactorial, and somewhat different for various birth cohorts. Nevertheless, because the obesity epidemic has serious implications for the health (Must, Spadano, Coakley, Field, Colditz, & Dietz, 1999
; Visscher & Seidell, 2001
), longevity (Olshansky et al., 2005
), economy (Wolf & Colditz, 1998
) and quality of life (Han, Tijhuis, Lean, & Seidell, 1998
; Hassan, Joshi, Madhavan, & Amonkar, 2003
) of the U.S. population, every effort should be undertaken to improve our understanding of its causes and potential solutions.