Researchers tracking the Massachusetts Male Aging Study measured the same men repeatedly across three survey waves spanning the late 1980s to the mid-2000s, comparing levels at a given age across different birth years.
Across more than 2,700 measurements taken on 1,532 men, average testosterone fell from one calendar period to the next by more than could be explained by each man simply growing older, and the drop held after accounting for weight, smoking and other health habits.
The size of the generational decline was larger than the decline normally attributed to aging itself, pointing to something in the environment or lifestyle of later birth cohorts rather than biology alone. The study could not identify a single cause, and the specific drivers, whether chemical exposure, obesity trends, or something else, remain an open question in later research.
Read the original study
Same age, lower testosterone, different decade.
Source
- Travison, T. G., Araujo, A. B., O'Donnell, A. B., Kupelian, V., & McKinlay, J. B. (2007). A population-level decline in serum testosterone levels in American men. The Journal of Clinical Endocrinology and Metabolism, 92(1), 196–202. https://doi.org/10.1210/jc.2006-1375