The Pentagon issued guidelines in September that make testosterone deficiency screening mandatory for male service members aged 30 and over. Fox News reported the details on Saturday.

Screening begins with an assessment of symptoms and risk factors. Further testing and treatment follow where a doctor judges it appropriate. Guidance covering hormonal health in female service members is still being written.

The reason the military is doing this is a number that has been falling for fifty years.

Admiral Brian Christine, a senior official at Health and Human Services and a urologist who spent more than thirty-five years in men’s health, says the average testosterone level for an American man in the mid-1970s was well over 650. He says it is now approaching 350.

That is close to half, inside one working lifetime.

“There is absolutely a national defense readiness ramifications for it,” Christine said. He also said the policy is not what critics will assume: “We’re not forcing testosterone on anybody.”

Dr. Jay Bhattacharya, the director of the National Institutes of Health, said the epidemiological trend is real and that researchers do not yet know what is causing it. “There are under-diagnosed testosterone deficiencies,” he said.

Bhattacharya also pointed to falling sperm counts as something that needs investigating. Christine raised environmental exposure, microplastics among them, as a possible factor. Neither man claimed to know the answer.

The recruiting figures are where this stops being abstract.

By Christine’s account, 77 percent of Americans between 17 and 24 cannot qualify for military service without a waiver. More than 70 percent of young men are ineligible. About 3 percent of young Americans say they are interested in serving at all.

Put those together and the pool of young men who both want to serve and are physically able to is small, and getting smaller.

Worth saying plainly what this report does and does not establish. It establishes a measured decline across decades and a military that has decided the decline is its problem now.

It does not establish why. The officials involved were careful about that, and the honest version of the story keeps their caution in it.