Where did Valentine's Day come from? (Think naked Romans, paganism, and whips.) What does it cost? And why do we fall for it, year after year?

Valentine's Day History: Roman Roots

More than a Hallmark holiday, Valentine's Day, like Halloween, is rooted in pagan partying. (See "Halloween Facts: Costumes, History, Urban Legends, More.")

The lovers' holiday traces its roots to raucous annual Roman festivals where men stripped naked, grabbed goat- or dog-skin whips, and spanked young maidens in hopes of increasing their fertility, said classics professor Noel Lenski of the University of Colorado at Boulder.

The annual pagan celebration, called Lupercalia, was held every year on February 15 and remained wildly popular well into the fifth century A.D.—at least 150 years after Constantine legalized Christianity in the Roman Empire.

Lupercalia was "clearly a very popular thing, even in an environment where the [ancient] Christians are trying to close it down," Lenski said. "So there's reason to think that the Christians might instead have said, OK, we'll just call this a Christian festival."

The church pegged the festival to the legend of St. Valentine.

According to the story, in the third century A.D., Roman Emperor Claudius II, seeking to bolster his army, forbade young men to marry. Valentine, it is said, flouted the ban, performing marriages in secret.

For his defiance, Valentine was executed in A.D. 270—on February 14, the story goes.

While it's not known whether the legend is true, Lenski said, "it may be a convenient explanation for a Christian version of what happened at Lupercalia."

Valentine's Day 2012: A Strengthening Economy?

Today's relatively tame Valentine's Day celebration is big business—the 2012 holiday is expected to generate $17.6 billion in retail sales in the United States. That's up from last year's $15.7 billion, according to an annual survey by the U.S. National Retail Federation (NRF).

The level of "discretionary spending" exhibited by survey results is "a strong indication our economy continues to move in the right direction," federation president Matthew Shay said in a statement.

That is, from the retailers' perspective, the fact that Americans are going shopping for candy, flowers, and jewels is a good sign for the economy.

But behavioral economics professor Dan Ariely said that suggesting that an uptick in Valentine's day spending is a sign of widespread recovery, "is premature, I think."

"There's a question of whether people are compensating," said Ariely of Duke University. "You could say that Valentine's Day is perfectly correlated with other expenditures in life, or you could say that Valentine's Day is compensating for other things."

Ariely, whose books include the bestseller Predictably Irrational: The Hidden Forces that Shape our Decisions, said there's good reason to splurge on Valentine's Day, even in a tough economy.

"If you treat yourself to something on next Thursday, just a random day of the year, there's an issue of whether it becomes a routine," he explained. "But if you splurge only at Valentine's Day, now your spending is more confined."

And spend Valentine's Day celebrants will, according to the retail federation survey. The average U.S. consumer is expected to shell out $126.03 on Valentine's Day gifts, meals, and entertainment—about $10 more per person than in 2011.

Spouses and significant others plan to invest $74.12 on Valentine's Day gifts for their significant other—up from last year's $68.98 average. Pets, however, are getting a little less retail love this year, with average planned spending on animal gifts down 52 cents to $4.52.

Why we go shopping at all on Valentine's Day, Duke's Ariely said, has a lot to do with herd mentality.

"Herds give us a sense of what is normative behavior—not normative in terms of rational but normative in terms of this is how people behave," he said.

On Valentine's Day, the normative behavior is to go out and spend money on things such as chocolate and flowers as an expression of love. So, when we ask ourselves what to do, "the answer is very simple," Ariely said.

Valentine's Day Cards

Greeting cards, as usual, will be the most common Valentine's Day gifts. Fifty-two percent of U.S. consumers plan to send at least one, according to the National Retail Federation survey.

The Greeting Card Association, an industry trade group, says about 190 million Valentine's Day cards are sent each year. And that figure does not include the hundreds of millions of cards schoolchildren exchange.

"Giving your sweetheart or someone [else] a Valentine's Day card is a deep-seated cultural tradition in the United States," said association spokesperson Barbara Miller. "We don't see that changing."

The first Valentine's Day card was sent in 1415 from France's Duke of Orléans to his wife when he was a prisoner in the Tower of London following the Battle of Agincourt, according to the association.

During the Revolutionary War, Valentine's Day cards—mostly handwritten notes—gained popularity in the U.S. Mass production started in the early 1900s.

Hallmark got in the game in 1913, according to spokesperson Sarah Kolell. Since then—perhaps not coincidentally—the market for Valentine's Day cards has blossomed beyond lovers to include parents, children, siblings, and friends.

Valentine's Day Candy: Cash Cow

An estimated 50.5 percent of U.S. consumers will exchange Valentine's Day candy in 2012, according to the retail federation survey—adding up to about a sweet billion dollars in sales, the National Confectioners Association says.

About 75 percent of that billion is from sales of chocolate, which has been associated with romance at least since Mexico's 15th- and 16th-century Aztec Empire, according to Susan Fussell, a spokesperson with the association.

Fifteenth-century Aztec emperor Moctezuma I believed "eating chocolate on a regular basis made him more virile and better able to serve his harem," she said.

But there's nothing chocolaty about Valentine's Day's most iconic candy: those demanding, chalky little hearts emblazoned "BE MINE," "KISS ME," "CALL ME."

About eight billion candy hearts were made in 2009, the association says—enough to stretch from Rome, Italy, to Valentine (map), Arizona, and back again 20 times.

What Is Love? Evolution and Infatuation

Valentine's Day is all about love. But what, exactly, is that?

Helen Fisher is an anthropologist at Rutgers University in New Jersey and author of several books on love, including Why We Love: The Nature and Chemistry of Romantic Love.

Fisher breaks love into three distinct brain systems that enable mating and reproduction:

Sex drive

Romantic love (obsession, passion, infatuation)

Attachment (calmness and security with a long-term partner)

These are brain systems, Fisher said, and all three play a role in love. They can operate independently, but people crave all three for an ideal relationship.

"I think the sex drive evolved to get you out there looking for a range of partners," she said.

"I think romantic love evolved to enable you to focus your mating energy on just one at a time, and attachment evolved to tolerate that person at least long enough to raise a child together as a team."

Valentine's Day, Fisher added, used to encompass only two of these three brain systems: sex drive and romantic love.

But "once you start giving the dog a valentine, you are talking about a real expression of attachment as well as romantic love."