For eons we poor humans have relied on trial and error to find our one true love, and thanks to poets and songwriters from Shakespeare to Norah Jones we know how well that's worked. But that was before the Internet.

Now, online dating sites are evolving into giant laboratories dedicated to solving the most profound human mystery of all - the key to lifelong love. Chemistry.com, eHarmony.com and PerfectMatch.com are among the latest generation of online dating sites that are staking their futures on the idea that they can use science to predict long-term romantic success for millions of singles looking for love.

Each has hired academics with curricula vitae as long as your arm, and created complex algorithms that try to match people according to temperament, interests, personality - and, yes, even genetics.

"Imagine if we could lower the divorce rate by 1 percent, all the lives that would be impacted by that," said Galen Buckwalter, a former professor of psychology who is vice president of research and development for eHarmony. It may take a generation to figure out whether any of the Web sites will succeed, but in the meantime they are collectively engaging in a social experiment of gargantuan size. Ultimately, they may prove whether or not science has a role in the most unscientific realm of all - love.

But the Web sites are working on that, too.

Anthropologist Helen Fisher, the scientific adviser to Match.com's offshoot, Chemistry.com, said that psychologists have understood for years which relationships work and which ones don't. People who share culture, economic status, education and religion fare much better than people who are different. All the dating services, including Chemistry.com, match for that, she said.

"Why do you fall in love with one person and not another?" said Fisher, a professor at Rutgers University. She thinks it's genetics.

Fisher cites the so-called "sweaty T-shirt experiment," a study in which women chose men with different immune systems based solely on the smell of their dirty T-shirts. The evolutionary theory behind their choice is that a mate with a different set of genes makes a baby with a stronger immune system.

"If women tend to be attracted to men with a different immune system, why would they not also be attracted to men with different serotonin, estrogen, dopamine and testosterone?" she said. "If you pool your DNA with someone with a different genetic profile, you create a wider array of genetics for the baby."

Genes for dopamine, for example, are associated with motivation and curiosity, she said. Genes for serotonin are linked to stability. Testosterone is related to drive and spatial abilities. She theorized that people have different levels of those body chemicals that put them into four broad personality types. The profile on Chemistry.com is designed to assign people to one of those four.

People who are well matched have complementary traits - a risk-taking explorer type, for example, would more likely be happy with a steady, predictable home-body type.

She's still figuring out whether it works. But the potential societal payoff is huge, she said.

There are some 33 million U.S. adult singles who go online, and about five to six percent of them subscribe to a dating website, according to the market research company JupiterResearch. Not only do they include the usual crop of young single people looking for love, but nowadays there are also all those aging, never-say-die baby boomers.

"Now we have Viagra, and estrogen and hip replacements, and those brain circuits can be triggered at any age," Fisher said.

PerfectMatch relies on a combination of similar and complementary traits to predict long-term compatibility, but not instant attraction.

"They spend 5 minutes on a date, and they know if there is chemistry," said Dr. Pepper Schwartz, the sociologist who developed the trade-marked Duet Compatibility Profiler for PerfectMatch. "What they don't know is how much this thing could go the distance."

The Duet Compatibility Profiler focuses on eight personality characteristics, including romantic impulsivity, energy and outlook. Compatible couples should match on four of them. They can either match or not on the other four. But each person should know themselves well enough to decide whether their mate should match on those characteristics or not.

eHarmony based its matching system on what 5,000 happily married couples had to say about why their relationships withstood the test of time.

"We never see two people who are exactly the same," Buckwalter said. "But in general if they are within a fairly consistent range in areas of personality, values and interests, those are the marriages that we found to be successful."

He admits that eHarmony's system so far does not identify what he called "the click," the spark that leads to romance instead of friendship. But, like Fisher and Chemistry.com, he's working on it. eHarmony recently launched their eHarmonyLabs, which it described as the first commercial research organization dedicated to the study of relationships - sort of a Bell Labs for the human heart. He plans to invite single people to eHarmonyLabs and observe them interacting with those of the opposite sex so he can study who is attracted to whom and why.

They have to do it in a lab because that spark only happens when two people meet face to face, just as it does in real life.

"They call it love at first sight, not love at first e-mail," Fisher said.