Why you're drawn to the blonde next door or end up sweating around your high school crush is, to some extent, a mystery. The so-called "spark" springs from a constellation of biology, psychology and lots of other -ologies (astrology, anyone?). But while chemistry may not be an exact science, math certainly is.

Figure that about 50 percent of marriages end in divorce; add to that the 10 to 15 percent of couples who permanently separate, plus the 7 percent who stay in unsatisfying unions and you're left with this depressing statistic: Only 3 in 10 couples end up happily married, writes Ty Tashiro, Discovery Network's relationship expert and University of Maryland professor, in his forthcoming book, "The Science of Happily Ever After: What Really Matters in the Quest for Enduring Love."

To shore up your chances of beating the odds, Tashiro plots a path to happily ever after, using analysis of studies about what makes love last and what's likely to bring about a romantic implosion. In short, focus less on chemistry and more on compatibility.

[Read: 'Sex and the City' Scribe on 'Life as a Wife.']

As he explains it, humans today are trapped by evolutionary urges that made for great survival instincts but provide little reward for modern love. Attraction to looks and money, which rank highly among men and women, respectively, may have worked to beget and support healthy offspring in the Stone Age. But those benchmarks don't pay off in developed societies, where common access to food, medicine and shelter have enabled 99 percent of folks to survive past the age of 35, Tashiro says. And because longer living means longer coupling, he suggests we rethink what we seek in a mate.

For one thing, the benefits of financial resources chiefly come by way of a base level of security - enough to remove the threat of distress - and high levels of income can have other hazards. For another, lust doesn't last.

[Read: How Money Affects Happiness.]

"Sometimes I am asked why infatuation and the feelings that accompany it, such as butterflies in the stomach or a racing heart, cannot last," Tashiro writes in his book. "These visceral feelings are powerful feelings of lust, and they cannot last for a simple reason; you would die." Such sensations equate to stress and high blood pressure that, should they continue unabated, could become toxic, he explains. "So, even though the lust component of being in love drives a very intense and visceral type of emotional experience, the intensity of passionate love is necessarily ephemeral."

While liking and lusting are both key features of romantic love, you're better off betting on the former, Tashiro says. According to research cited in his book, lust declines at a rate of 8 percent per year of marriage, while liking declines at a rate of 3 percent. Moreover, specific personality traits are likely to predict marital success - and failure - in the long run. And since you can't have it all - it's a mathematics impossibility, he says - it's best to pick based on personality. That's not to throw sex out the window - it's fundamental for a successful marriage. (Quoting one of his graduate school advisers, Tashiro writes, "If your partner is bad at tennis, it's not a big deal, because you can go play tennis with someone else. If your partner is bad in bed, well, that's a big deal.")

Ideally, if you can match up with someone based on three personality traits - which he calls agreeableness, lack of neuroticism and lack of seeking novelty - you're more likely to have your bases covered, sexual innuendo and all.

But is limiting yourself to seeking just three qualities in a partner a bit, well, limiting?

Not to Tashiro, who finds this approach more hopeful and attainable than the unrealistic standards that can derail people's hopes for lasting love. "There's been no other time in history or a culture where we've put so many demands upon one person to be so many things," he tells U.S. News. Adjusting expectations might enable greater appreciation of one's partner, he says. "People might understand, too, what's lucky," he says. "I think when you see these marriages that really function very well, there's this great sense of gratitude almost on a daily basis."

Jeannie Assimos, director of content for eHarmony's blog, agrees about the importance of compatibility. "The most important thing is to know what you want and know what you can and can't deal with," Assimos says, recommending that singles aim for a list of 10 qualities they'd like in a partner in terms of beliefs and values. "The more similar you are, it's just easier - you get each other more." But that's not say you should sacrifice chemistry, she says. "You have to have that."

But what exactly even is chemistry? Physical attraction plus a certain connection, explains sex and relationship therapist Laura Berman, author of "Loving Sex: The Book of Joy and Passion." "It's a mysterious and beautiful formula that you can't necessarily force or create," and one that's critical for a lasting union, she says. Still, "I think the most important quality that you need in a mate who's going to go for the long haul with you is someone who is open to learning about themselves and willing to change and be flexible and who is resilient - because you're going to have crises, you're going to have conflicts, you're going to look at things differently, you're going to go through hard times in life and you have to be able to communicate."

[Read: How to Be a Good Listener.]

Rachel Pomerance Berl is a Health + Wellness editor at U.S. News. You can follow her on Twitter @RachelPomerance, circle her on Google+, subscribe to her articles or email her at