After weeks apart, you and your partner are anxious to catch up. And we're not just talking about conversation.

The concept is simple enough: Abstinence makes the heart — and the rest of the body — grow fonder. But why? Is reunion sex simply the emotional and physical equivalent of taking that first sip of cool water after a long walk in the desert? 

Maybe not. Partners who have been apart may be in fact be genetically programmed to have more sex and be more strongly attracted to each other when they reunite, according to some researchers. It's all about sperm competition, apparently. 

Men are driven to displace the competing sperm of any men who may have slept with their partner while the two were apart, according to sex researcher Todd Shackelford, of Florida Atlantic University. And while that may not be a very romantic notion, some researchers say it makes evolutionary sense. 

"It's virtually impossible to guard one's partner 100 percent of the time," said Shackelford. "Over human evolutionary history, until very recently, men could never be certain 100 percent of the time that the woman's offspring is their own." 

So, men compete to impregnate women, with the "winner" passing on his DNA to future generations. The primary goal of sex, after all, is to have your genes live on. 

 
'Over human evolutionary history, until very recently, men could never be certain 100 percent of the time that the woman's offspring is their own' — Todd Shackelford 
 

Men certainly seem more affected by the phenomenon. In one study, participants were asked to list how long they had been apart from their mate, and to rate their attractiveness and desire for sex on a 10-point scale. The study found women's desire did not much change in relation to time spent apart. But men's interest in sex rose about one point for every 100 hours apart; and ratings of their mates' sexiness rose a half-point. 

Are pent-up sexual urges to blame? Shackelford said no: The surge of libido isn't related to the last time the man had sex with his partner, only the amount of time the two have spent apart. And, he noted, prior research shows the longer a couple is separated, the higher the man's sperm count. 

Reunion could be the saving grace of the long-distance relationship, geography-impaired couples agree. Scott, 25, was separated from his long-term girlfriend for months at a time when she lived in Ithaca, N.Y. His one-word description of their reunions: "Vigorous." 

For Ben and Sylvia, who were separated for about two years while attending colleges in Chicago and Oregon, respectively, it also served the purpose of re-establishing the relationship. 

 
'There's something a little bit nervous about it, because you haven't been with them — they might look different, or act different than you remember' — Sylvia 
 

"While you're apart, you have to fill in your own ideas, because you don't have the person there with you," Ben said. "But when you get back together, it feels weird that your fantasies weren't quite on. Then you realize what the person is really like, and you get re-acclimated. Reunion sex is good for that initial first contact, like, 'Hey, I remember you!'" 

There's also the fear the relationship won't be the same — which can even serve to spice things up. 

"There's something a little bit nervous about it," said Sylvia, "because you haven't been with them — they might look different, or act different than you remember. 

But it's not the same for everyone. Suzy, who lives in Kansas City while her boyfriend lives on the East Coast, says reunion sex is not a big deal. She says their libidinous encounters are always turned up to 11. 

"He's consistently passionate, and I'm always attracted to him, so it's pretty much constant." 

— Reuters contributed to this report