The emails usually begin with "Dear Sir/Madam," says Bailey Johnson at CBS News. "And then come the promises of quick money, the assurances that a royal family member will make good on the bargain (accompanied by numerous typos)." The Nigerian email scam has become such a familiar feature of the internet era that it has its own extensive Wikipedia page. With such heavy exposure, it's puzzling that the scammers continue to send out their predictable messages. What recipient would still believe that he's just a few clicks away from receiving a fortune from a Nigerian prince? However, there is actually a solid mathematical method behind the complete obviousness of the Nigerian email scam, says Corman Herley, a Microsoft researcher. Here, a guide to Herley's findings:

What is Herley's rationale?
The emails virtually cry out: "This is a scam!" As a result, says Herley, only the most gullible people will actually contact the scammer and inquire about the vast reservoir of gold or the millions of dollars in oil profits being advertised. The vast majority of people will delete the email or report it as spam. "By sending an email that repels all but the most gullible," says Herley, "the scammer gets the most promising marks to self-select, and tilts the true to false positive ratio in his favor."

How does that help the scammers?
The scammers don't want to deal with smart people, since the scheme unfolds over a series of exchanges that require a precious investment of time. After email contact is made, the scammer has to persuade the respondent to part with a checking account or credit card number. Even a fairly gullible person would begin to get suspicious, and could pull out. By weeding out everyone but the most gullible, the scammers can maximize efficiency.

What's the best way to weed out the intelligent?
The word "Nigeria." It sets off alarm bells for most people. And even though most Nigerian email scams are actually run out of American and Europe, they do have strong ties to the country itself. In fact, so many of the emails come from the country that they're sometimes referred to as 419 frauds, after a "section of the Nigerian criminal code that covers such scams," says Will Oremus at Slate. "Why Nigerians become email scammers in the first place is, perhaps, a question for another paper."

Sources: CBS News, Microsoft, Slate