The "Nigerian prince" email scam is perhaps one of the longest-running Internet frauds. Actress Anne Hathaway even joked about it in her monologue on "Saturday Night Live" over a decade ago. Also called "Nigerian letter" scams or "foreign money exchanges," these typically start with an email from someone overseas who claims to be royalty. The fraudsters lure you in by offering a share of a huge investment opportunity or a fortune they can't get out of the country without your help. Then they ask you either for your bank account number so they can transfer the money to you for safekeeping, or for a small advance payment to help cover the expense of transferring the money. That's when they either take your payment and disappear, or, worse, drain your bank account. Americans lost $703,000 last year to these types of frauds, according to a new report by ADT Security Services, using data from the Better Business Bureau's Scam Tracker. "As long as these types of scams keep working, people will continue to use them," Anja Solum, ADT project manager, tells CNBC Make It. Over the past three years, ADT calculated that Nigerian letter-style scams have cost victims an average of $2,133.

Why people fall for these scams—and how to stay safe

The reason these scams are so effective is that they present victims with a "perfect storm of temptations," Dr. Frank McAndrew, a social psychologist and professor at Illinois-based Knox College, tells CNBC Make It. First, these scams play on people's greed. Many times, the scam is set up in a way where victims are promised that they'll make a hefty financial profit without much effort, McAndrew says. In most successful scams, the fraudsters also prey on your desire to be a hero. "We get the opportunity to feel good about ourselves by helping another person in need," McAndrew says. "After all, what could be more noble than helping an orphan in need or helping some poor soul recover money that rightfully belongs to them in the the first place?"

As long as these types of scams keep working, people will continue to use them. Anja Solum ADT project manager

The best way not to fall for these is to recognize them for what they are. Experts say these types of emails are typically unexpected and from an unknown sender. Some email providers may even automatically send these to your spam folder. If this type of email does land in your inbox, though, don't send money or give out your personal information to strangers, no matter how sad the story or enticing the reward. If you do fall for scams like these, McAndrew adds, don't feel too bad: Remember that lots of other people have made the same mistake. "The scammers are also good at luring us into a relationship before the sting occurs, which simultaneously builds up a sense of trust, which then causes us to almost feel a sense of obligation to provide help to them when the need arises," he says.

These scams are expensive, but they're not the worst

Overall, Americans lost over $26 million to scams last year, according to ADT. While the Nigerian prince-style schemes can cost a lot if you fall for them, investment fraud and romance scams are the most expensive for victims. Investment fraud of all types, including Ponzi and pyramid schemes, committed over the past three years cost victims an average of $8,648, according to ADT. Victims of romance schemes, sometimes referred to as "sweetheart scams," lose an average of $6,003.

Click to enlarge Sweetheart scams are similar to foreign money exchange schemes, but they generally start within online dating websites as opposed to by email. Fraudsters create fake dating profiles and pose as users looking for love. They spend time talking and getting to know their victims. Once there's a level of trust built up, they ask for money and play on victims' emotions. Many times, these scams target the elderly.

The most common type of scams