On a recent Saturday morning, Craig Adams stood outside the Robert Wood Johnson University Hospital in New Brunswick, New Jersey. It was sunny but cold. Adams, who had turned 40 the day before, wore white sneakers and a black T-shirt over a long-sleeve shirt. A fuzz of thinning hair capped his still-youthful face. His appearance would have been unremarkable if not for the red splotch of fake blood on the crotch of his white trousers. The stain had the intended effect: drivers rounding the corner were slowing down just enough to see the sign he was holding, which read "No Medical Excuse for Genital Abuse".

Next to him, Lauren Meyer, a 33-year-old mother of two boys, held another sign, a white poster adorned only with the words: "Don't Cut His Penis". She had on a white hoodie with a big red heart and three red droplets, and a pair of leopard-print slipper-boots to keep her feet warm for the several hours she would be outside. Meyer's first son is circumcised; she sometimes refers to herself as a "regret mother" for having allowed the procedure to take place.

It was two days after Christmas. Adams and Meyer had each driven about an hour to stand by the side of a road holding up signs about penises. On that same day, a woman stood alone at what qualifies as a busy intersection in the small town of Show Low, Arizona. She also wore white trousers with a red crotch, and held aloft anti-circumcision signs. A few people more people did the same in the San Francisco Bay area.

The protests were triggered by a recent event, but the issue at stake was an ancient one. Circumcision has been practised for millennia. Right now, in America, it is so common that foreskins are somewhat rare, and may become more so. A few weeks before the protests, the Centers for Disease Control and Prevention (CDC) had suggested that healthcare professionals talk to men and parents about the benefits of the procedure, which include protection from some sexually transmitted diseases, and the risks, which the CDC describes as low. But as the protesters wanted drivers to know, there is no medical consensus on this issue. Circumcision isn't advised for health reasons in Europe, for instance, because the benefits remain unclear. Meanwhile, Western organisations are paying for the circumcision of millions of African men in an attempt to rein in HIV – a campaign that critics say is also based on questionable evidence.

Men have been circumcised for thousands of years, yet our thinking about the foreskin seems as muddled as ever. And a close examination of this muddle raises disturbing questions. Is this American exceptionalism justified? Should we really be funding mass circumcision in Africa? Or by removing the foreskins of men, boys and newborns, are we actually committing a violation of human rights?

The tomb of Ankhmahor, a high-ranking official in ancient Egypt, is situated in a vast burial ground just outside Cairo. A picture of a man standing upright is carved into one of the walls. His hands are restrained, and another figure kneels in front of him, holding a tool to his penis. Though there is no definitive explanation of why circumcision began, many historians believe this relief, carved more than four thousand years ago, is the oldest known record of the procedure.

The best-known circumcision ritual, the Jewish ceremony of brit milah, is also thousands of years old. It survives to this day, as do others practised by Muslims and some African tribes. But American attitudes to circumcision have a much more recent origin. As medical historian David Gollaher recounts in his book Circumcision: A History of the World's Most Controversial Surgery, early Christian leaders abandoned the practice, realising perhaps that their religion would be more attractive to converts if surgery wasn't required. Circumcision disappeared from Christianity, and the secular Western cultures that descended from it, for almost two thousand years.

Then came the Victorians. One day in 1870, a New York orthopaedic surgeon named Lewis Sayre was asked to examine a five-year-old boy suffering from paralysis of both legs. Sayre was the picture of a Victorian gentleman: three-piece suit, bow tie, mutton chops. He was also highly respected, a renowned physician at Bellevue Hospital, New York's oldest public hospital, and an early member of the American Medical Association.

After the boy's sore genitals were pointed out by his nanny, Sayre removed the foreskin. The boy recovered. Believing he was on to something big, Sayre conducted more procedures. His reputation was such that when he praised the benefits of circumcision – which he did in the Transactions of the American Medical Associationand elsewhere until he died in 1900 – surgeons elsewhere followed suit. Among other ailments, Sayre discussed patients whose foreskins were tightened and could not retract, a condition known as phimosis. Sayre declared that the condition caused a general state of nervous irritation, and that circumcision was the cure.

His ideas found a receptive audience. ...