The Secular Trend of the Body-Mass-Index of the U.S.-Born Population, 1882-2021, the Rise of an Obesogenic Environment, and the Decline in Willpower

We employ a generalized additive model with complexity-penalized splines to estimate the trend in BMI values of both adults and children, using data in the 18 cycles of the NHANES surveys. To ensure a consistent obesogenic environment across survey participants, we restrict the sample to U.S.-born individuals and emphasize birth-cohort effects rather than period effects, prevalent in the literature. This choice reflects the premise that an individual’s weight at any time is shaped by cumulative exposures beginning at birth. Birth cohorts are more likely to share comparable social, cultural, technological, and economic environments across the life course, unlike measurement cohorts. Our findings indicate that BMI values rose throughout the 20th century, albeit at varying rates. The annual rate of increase among those born in the 1920s was close to 0.1 unit but the trend decelerated during the Depression and war, reaching a trough in the 1940s or early 1950s. Black females are an exception to this generalization, since their BMI values continued to increase at 0.1 per annum rate throughout the first half of the century. Those born in the 1950s and 1960s experienced an acceleration of their BMI values, coinciding with the emergence of an increasingly obesogenic environment and decline in willpower, a latent variable that, to our knowledge, remains underexplored in the obesity literature. We also document the tapering of BMI values by the late 20th century and plateauing among children in the 21st century, consistent with the conjecture that the obesogenic environment may have reached a saturation point.