Studies show that men’s testosterone levels have been declining for decades. The most prominent, a 2007 study in the Journal of Clinical Endocrinology and Metabolism, revealed a “substantial” drop in U.S. men’s testosterone levels since the 1980s, with average levels declining by about 1% per year. This means, for example, that a 60-year-old man in 2004 had testosterone levels 17% lower than those of a 60-year-old in 1987. Another study of Danish men produced similar findings, with double-digit declines among men born in the 1960s compared to those born in the 1920s.