The Trump administration seems to harbor a hulking obsession with testosterone. Health Secretary Robert F. Kennedy Jr. once claimed that the president’s testosterone levels are unusually high for a man of his age, and Kennedy is known to take a supplementary dose of the male sex hormone himself. In April, the FDA moved to expand access to testosterone-replacement therapy for greater numbers of men. And last month, Defense Secretary Pete Hegseth announced that “warfighters” older than 30 will have their testosterone levels checked annually so that they can stay on the “leading edge of lethality.” The need for expansive testing or for testosterone replacement is questionable, according to several medical experts I spoke with. But underneath the administration’s preoccupation lies a real trend: Although most men’s levels are still within the normal range, average testosterone has been declining among men for the past several decades. A recent review of studies involving more than a million men all over the world found that average testosterone levels in blood have dropped by about 20 percent over the …