Travison et al. 2007 — A population-level decline in serum testosterone in American men

What the study found

Drawing on the Massachusetts Male Aging Study across three survey waves between 1987 and 2004, the authors reported a decline in average serum testosterone among American men that persisted even after accounting for the men's age, and that did not appear to be explained by measured changes in their health and lifestyle. ^[1]

an age-independent population-level decrease in [testosterone] in American men

^[1]

The authors left the cause unresolved, noting that possible birth-cohort or environmental effects were "not captured in observed data."

Why it matters here

This study is frequently invoked in claims that testosterone is collapsing among men. It does document a genuine population-level decline that is not merely an artefact of aging, which is why it is cited. At the same time, the authors themselves describe the underlying cause as unresolved rather than established, so the paper supports the existence of a downward trend without endorsing any single explanation for it.