Research Outline

Decline in Male Testosterone Levels


To determine if male testosterone levels are lower now than a generation ago and, if so, identify the causes.

Early Findings


  • A study published in 2007 finds a "substantial" decline in male testosterone levels since the 1980s in the United States. The study, however, finds no relationship between this trend and age.
  • While the reasons are unclear, the researchers of the study hypothesize that the increase in obesity and decrease in cigarette smoking might be potential explanations as being overweight is associated with lower testosterone levels and smoking is associated with higher testosterone levels.
  • The researchers also suggest that some environmental exposure might be responsible for the decline as well, while stressing that the matter is complex and there is likely not one cause solely responsible for the decline.
  • A publication by the Journal of Clinical Endocrinology & Metabolism also cites other lifestyle trends, from increased temperatures in homes and offices to lack of exercise and tight underwear, as potential causes.
  • A more recent publication by Nature Ecology and Evolution suggests that average levels of testosterone are linked to where men live with those living in richer environment tend to have higher testosterone levels.
  • Two additional studies presented at the Scientific Congress of the American Society for Reproductive Medicine (ASRM) in Denver find a decline in sperm count and sperm motility of American and European men in the past decade. The finding is said to be consistent with other findings in the past few decades.
  • While one of the studies finds no single or definitive cause, the other suggests that “chemical exposures or increasingly sedentary lifestyles” could be potential links.


Observational studies have found a declining trend in male testosterone levels over the past few decades. While changes in lifestyle are cited as potential causes, researchers agree that there is not one definite cause due to the complexity of the subject matter.