IN FAULTY English, the e-mail describes vast riches in search of an owner. Your new pen pal just needs your bank account to park the money—and will pay richly for the favour. In fact, the fraudsters will empty your account, or sucker you into paying fees for cash that never materialises.

Though Nigeria is well known as the source of such tricks (called 419 scams after the relevant paragraph in that country's criminal code), many crooks make puzzlingly little attempt to hide their origins. In a new paper*, Cormac Herley of Microsoft Research has used maths to show why: blatancy is a means of weeding out all but the most credulous respondents.

He argues that scammers are rational actors. A big cost for them is the time they spend coaxing fully into their net those who show initial interest. So they need to select the most promising targets, rather than timewasters or the wary. “By sending an e-mail that repels all but the most gullible, the scammer gets the most promising marks [victims] to self-select,” he says. But the absurd stories and rum prose are not necessarily works of art. Scammers may simply reuse the e-mails that work best.

Officials are intrigued by Mr Herley's conjecture. According to Basil Udotai, who was formerly the cybersecurity director at the office of Nigeria's National Security Adviser, “There are more non-Nigerian scammers claiming [to be] Nigerian than ever reported. Even when Nigerians relocate to other West African countries they retain the Nigerian status, addresses and operational bases in their e-mails for competitive reasons.” But he suggests that this has another motive: it is Nigeria's dreadful reputation for corruption that makes the strange tales of dodgy lawyers, sudden death and orphaned fortunes seem plausible in the first place.

Jenna Burrell, who studies online scams and is at the University of California, Berkeley, says many African fraudsters are now pretending to be non-African to look more credible. Eve Edelson, the author of “Scamorama”, a book about baiting the fraudsters, says African criminals use many methods: not only fake-sounding e-mails but realistic ones, too.

But some empirical evidence backs up Mr Herley's broader point about the fraudsters' incentive to filter out discerning targets. Eric Park of Symantec, a computer-security firm, notes that advance-fee scams that purport to offer cheap loans have begun to change from appearing very official and asking for information, to looking amazingly louche. That suggests that these scamsters too are trying to attract idiots, rather than serious-minded borrowers.

One implication of Mr Herley's work is that a little bit of public-spirited scam-baiting—wasting the fraudsters' time by pretending to be a potential victim—can increase the scammers' costs and undermine their business model. For those with some time to spare, joining a cyber-posse may offer an amusing way to make the world a safer place.

* “Why do Nigerian scammers say they are from Nigeria?” research.microsoft.com/pubs/167719/ WhyFromNigeria.pdf