"It's more common the younger you are, so it's probably increasing in incidence," said Basil Donovan the head of the Sexual Health Program at the UNSW's Kirby Institute, who was another author of the study.

Perhaps the prisoners were more likely to be Asian because penile implants have existed in Asian prisons since the 18th century. The Journal of Sexual Medicine article documents how the Yakuza placed one nodule, preferably a pearl, under the skin of the foreskin for every year they were confined. "In this way, he underlines on the one hand the affiliation to his caste, on the other hand the Yakuza practices the implantations to recompense his mistress with increased sexual stimulation for his absence," the authors wrote. One study from the eighties, mentioned in both Plos One and The Journal of Sexual Medicine found that 22 percent of prisoners in a Japanese detention center had penile implants and most of them were members of the Yakuza. Some prisoners had up to 20 nodules in their penis. The practice appears to be established in detention centers throughout Asia, including parts of Russia, and Eastern Europe, where penile implants are sometimes called sputniks. Like the Yakuza, prisoners pledge allegiance through the ritual, which also occurs in Russian army units.

Despite the common claim that penile inserts increase a partner's sexual pleasure, The Journal of Sexual Medicine found that there is little to suggest that this is the case. Women including wives and sex workers reported finding the beads to be uncomfortable and in some cases they caused bleeding and abrasion. Most sexual partners found that the inserts made penetration difficult or caused infection. Penile implants were also found to be a threat to effective contraception as condoms were less likely to fit and more likely to break.

Still, with sexual satisfaction in mind, the practice made its way to U.S. prisons. A separate paper published online in The Journal of Sexual Medicine, in 2011, documented three Hispanic men, from three different correctional facilities in the southwestern United States, who had all presented with penile skin infections after inserting carved domino pieces under the skin of their penises. In two of the cases, they were told that it would enhance the pleasure of a female partner.

One of the men, a 25-year-old from a Texan prison, placed a heart carved out of a domino through an incision into his mid-dorsal foreskin that he made with the tip of a ballpoint pen. "Given similarities between the incidents, it is reasonable to speculate that penile modification by self-placement of foreign bodies is not a rare practice among some members of the incarcerated population in southwest United States prisons," the authors wrote. "Given the circumstances under which such devices are placed, it seems unlikely that men who attempt this procedure will be able to realistically avoid infectious complications."