The secular trend of the body-mass-index of the U.S.-born population and the rise of an obesogenic environment, 1882–2021

• BMI of birth cohorts rose throughout the 20th century with three structural breaks. • BMI values accelerated with the rise of the obesogenic environment in the 1950s–60s. • Evolutionary mismatch exists between the brain’s dopamine system and the obesogenic environment. • BMI increases have tapered in the late 20th century and even plateaued in the 21st century. • The obesogenic environment has likely reached a saturation point. We estimate trends in BMI values by birth cohorts (1882-2021), stratified by ethnicity and gender using nationally representative survey data on U.S.-born individuals. BMI growth accelerated among cohorts born in the 1920s as public health improvements reduced infectious disease burdens, and again among cohorts born in the 1950s and 1960s with the emergence of an increasingly obesogenic environment. An obesogenic environment is characterized by the pervasive availability and affordability of caloriedense foods, oversized restaurant portions, excessive exposure to aggressive food marketing, and high density fast-food outlets. By the late twentieth century, BMI increases tapered at historically high levels and plateaued among children in the twenty-first century, suggesting that the obesogenic environment approached saturation. These patterns indicate that weight gain reflects persistent environmental change rather than individual choices alone. Early-life exposure to these conditions is associated with higher body weight throughout adulthood, highlighting obesity as a structural and intergenerational phenomenon.