Believers in the quest to make America great again should consider where there is evidence things are going down the tubes. There is hardly a more dismal example than the national decline in sperm production. Last week, scientists published a study confirming that sperm counts are half what they were in the early 1970s — and not just in America, but in Europe, Australia and New Zealand, too. The more alarmist accounts warned that the human race is teetering on the brink of extinction.

The good news is that our extinction is probably about as imminent as that of cockroaches. But the bad news is that something disturbing is going on. Some scientists worry that the ubiquitous hormone-disrupting chemicals used in plastics, cosmetics and food processing could affect human reproductive health. In doses comparable to typical human exposures, some of these so-called endocrine disrupters will cause a host of problems in lab rats, including lowered sperm counts.

But the good news first. Human fertility is not declining and the human population is still growing fast, despite those dwindling sperm numbers. That’s because humans make a superabundance of sperm — about 200 million per ejaculation — which is 199,999,999 more than the minimum you need to make a baby. Of course, the female reproductive tract is hard to navigate, so it is advantageous to launch a large fleet. But biologists say we owe our downright extravagance to something they call sperm competition.

Scientists have documented this phenomenon in a number of species, including insects and mice. There is still some disagreement about how much it affects humanity, but those who make a case for human sperm competition see evidence in the ways our physiology compares with that of our closest primate relatives.

In chimpanzees, for example, females will mate in a seemingly indiscriminate fashion when they enter a two to three-day fertile period each month, said Gordon Gallup, an evolutionary psychologist at the University of Albany. That is how sperm from different males wind up in the same place at the same time, thus setting up a competitive tournament in the female Fallopian tubes. Male chimps compete with a numbers game, ejaculating an average of 600 million sperm, Gallup said, which they produce in “pendulous” testicles three times the size of human ones.

In gorillas, by contrast, females primarily mate only with the alpha male, whose sperm therefore do not have to face much competition. So, predictably, gorillas have very small testicles and much lower sperm counts.

Humans are in the middle. Monogamy is common among human beings, but so is cheating. Todd Shackelford, an evolutionary psychologist at Oakland University in Michigan, said there have been a number of studies of human paternity showing between 1.9 per cent and 5 per cent of children are not the offspring of the purported father. So it appears men, too, have evolved for sperm competition. “It’s like playing the lottery,” he said. “The more tickets you buy, the greater your chances of winning.”

Evolution probably has not made human sperm counts plummet in just a few decades — especially since cheating has not gone out of fashion. That suggests an environmental cause is more likely. Some have put the blame on stress, sedentary work habits, even the popularity of “tighty whities”. Others have focused on a class of substances called phthalates, which are in used in flexible plastics, perfumes, hairspray and nail polish.

The leader of the most recent sperm study, Shanna Swan of the Icahn School of Medicine at Mount Sinai, said while they cannot prove phthalates are contributing to sperm decline, both human and animal studies offer reason for concern. Exposing lab animals to phthalates causes an increase in a birth defect called hypospadias — an incomplete development of the male genitalia, leading the urethra to exit on the underside of a boy’s penis. In humans there is some concern that the rate of hypospadias is rising, although it is up for debate.

Another sign of disrupted male development is a shortening of what the scientists call the anogenital distance, which is, as it sounds, the distance between the anus and the genitals. Normally in both humans and lab rodents, this distance is twice as long in males as females. In lab mice exposed to phthalates, the male AGD is shortened, and studies in humans are showing a connection between phthalate concentrations in pregnant women’s urine and a shorter AGD in their sons. Swan says other studies show that men with a short AGD, hypospadias or undescended testicles are more likely to have low sperm count, infertility and cardiovascular disease. These problems are all connected.

Why are males affected? It comes down to the quirks of mammalian sexual development. Male and female embryos look alike from the start, with little nubs of tissue capable of becoming the genitals of either sex. For humans, the sexes differentiate in a window between the eighth and fifteenth week of the mother’s pregnancy, when male hormones should start sculpting the male organs. Females require some hormones for their development, but their pathway is not so easily disrupted.

So if Donald Trump decides he wants to make American ejaculation great again, he might start by increasing efforts by the Environmental Protection Agency to identify the most hazardous endocrine disrupters and issue warning labels. There are tens of thousands of unregulated chemicals out there, but only a subset of them show the potential to interrupt male development. Trump, as the first presidential candidate to use the size of his genitals as a debating point, should understand why the issue matters.

• Faye Flam is a Bloomberg View columnist. She was a staff writer for Science magazine and a columnist for the Philadelphia Inquirer, and she is the author of The Score: How the Quest for Sex Has Shaped the Modern Man