ON SOME Swedish trains, passengers carry their e-tickets in their hands—literally. About 3,000 Swedes have opted to insert grain-of-rice-sized microchips beneath the skin between their thumbs and index fingers. The chips, which cost around $150, can hold personal details, credit-card numbers and medical records. They rely on Radio Frequency ID (RFID), a technology already used in payment cards, tickets and passports.

By one estimate there are 10,000 cyborgs with chip implants around the world. Sweden, home to several microchip companies, has the largest share. Fifty employees of Three Square Market, a Wisconsin-based firm, volunteered to receive chip implants that can be used to pay at vending machines and log in to computers. Individuals can order do-it-yourself kits, which come with sterilisation tools and a needle to inject the device, or attend “implant parties”, where a professional gives chips to a group. Sometimes they get T-shirts that say “I got chipped”.

Jowan Österlund, the founder of BioHax, a Swedish firm, claims chips are more secure than mobile phones because they are hard to hack. But sceptics still have concerns. RFID chips do not have GPS, but they leave a digital trail when they interact with doors, printers or turnstiles. In 2004 the Mexican attorney-general and his staff had chips inserted in their arms that tracked who had accessed sensitive information.

So why take the risk? Convenience is one draw. The infrastructure for microchip use exists wherever contactless IDs or payments are accepted. Sweden is well suited, as the world’s second most cashless country (after Canada).

But the chips have little use unless companies play along. Few shops recognise chip implants yet. Even those organisations that do have had teething troubles. When Swedish rail officials began scanning passengers’ microchips, they saw LinkedIn profiles rather than evidence of ticket purchases. For now the chips are used largely as digital business cards, substitutes for keys or to store emergency documents such as wills.

So exhibitionism is another explanation. Chip enthusiasts include followers of a “transhumanist” ideology that seeks to optimise human bodies with technology. Elon Musk, an American entrepreneur, has invested in tech that merges machines with human brains. Some Christians, meanwhile, fear that microchips are “marks of the beast” foretold in the Bible. Hardly, says Mr Österlund. After all, “people once thought the Beatles were the Antichrist.”