Men damage their fertility during efforts to make themselves "look more attractive"
A lot is said and written about female fertility these days, and we are forever being warned about declining rates, mostly as a result of more and more women delaying motherhood until their 30s and 40s.
However, it turns out, modern men are also at rik of damaging their fertility – from trying to look good.
Yes, that's right. Scientists have recently uncovered an evolutionary paradox where men damage their ability to have children during efforts to make themselves look more attractive.
Named the Mossman-Pacey paradox after the scientists who first described it, experts say that the issue here seems to be taking steroids to get a buff physique or anti-baldness pills to keep a full head of hair, as both of these have been shown to damage fertility.
"I noticed some men coming in to have their fertility tested and these guys were huge," says Dr James Mossman, now at Brown University in the United States, to the BBC.
"They are trying to look really big, to look like the pinnacles of evolution. But they are making themselves very unfit in an evolutionary sense, because without exception they had no sperm in their ejaculation at all."
Mossman was studying for his doctorate in Sheffield when he made the connection with steroid abuse, which are often used by bodybuilders to buff up their appearance.
Anabolic steroids mimic the effect of the male hormone testosterone in the body and are used as performance-enhancing drugs to increase muscle growth.
The steriods fool the brain's pituitary gland into thinking the testes are going into overdrive, so the glands react by shutting down the production of two hormones - called FSH and LH - which are the key hormones that drive the production of sperm.
"Isn't it ironic that men go to the gym to look wonderful, for the most part to attract women, and inadvertently decrease their fertility," asks Prof Allan Pacey, from the University of Sheffield. "I would say more anabolic steroid users are likely to become sterile than you would think - 90% probably."
The two researchers also say there is a similar theme in men using medication to prevent male pattern baldness, as the drug Finasteride changes the way testosterone is metabolised in the body and can limit hair loss, but side effects can include erectile dysfunction and a hit to fertility.
"Baldness is a bit more hit-and-miss," says Pacey. "But sales are going through the roof and that makes it an increasingly common problem."