A testosterone level decline has occurred in all age groups of men since 1987. There were a total of 2769 observations of 1532 men between 1987-89, 1995-97 and 2002-04.
While this study did not recognize a cause, obesity which is up over the decades can cause a decrease in testosterone; and cigarette smoking, which is down over the decades, can cause an increase in testosterone.
Obesity only accounted for a small percentage as a factor in the testosterone decline. Body Mass Index (BMI) was used to measure body fat, which is not as accurate a measurement of body fat compared to underwater weighing or bioimpedance methods.
Travison TG, Araujo AB, O’donnell AB, Kupelian V, McKinlay JB. A population-level decline in serum testosterone levels in American men. J Clin Endocrinol Metab. 2006 Oct 24