Scientists have uncovered a paradox where men try to make themselves into "pinnacles of evolution" while simultaneously becoming "evolutionary duds." "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?" says Allan Pacey, one of two scientists behind the Mossman-Pacey paradox, as described in the Journal of Internal Medicine. His research partner, James Mossman, was studying for his doctorate at the UK's University of Sheffield when he noticed "huge" men requesting fertility tests, per the BBC. He soon linked their massive physique to anabolic steroids—a so-called "vanity-based medication" that in addition to increasing muscle growth causes the brain's pituitary gland to end production of key hormones driving sperm production.
"They are trying ... to look like the pinnacles of evolution. But they are making themselves very unfit in an evolutionary sense," says Mossman. "They had no sperm in their ejaculation at all." Newsweek describes side effects including shrunken testicles and erectile dysfunction, as Pacey estimates 90% of anabolic steroid users are at risk of becoming sterile. Yet "the message is not getting out to young men that it's a problem and a bit of info could save them a lot of heartache." The researchers found a similar pattern among men using anti-baldness medications, which can also cause erectile dysfunction. At this point, they believe the paradox is exclusive to humans. Per the BBC, some birds choose not to reproduce and instead raise siblings' offspring, but half of the bird's genes are still passed on. (Sperm counts have dropped rapidly in recent years.)