Valentine's Day is the day of love, but is love the missing link in modern happiness? Consider:

Longevity, education, and standards of living have all risen spectacularly in the United States in recent decades -- and yet so has depression, which is 10 times more common today than 40 years ago. Since John Kenneth Galbraith wrote his salutation to the American achievement of permanent prosperity, "The Affluent Society," in 1957, real income for the typical American has almost tripled. Yet the percentage of Americans who describe themselves as "happy" in the long-running National Opinion Research Center has not budged in the same period. We can all think of other examples of people being discontent, stressed, or even flat-out miserable when, by physical circumstances at least, life seems pretty darn good for the majority of Americans.

What's going on here? Thomas Lewis, Fari Amini, and Richard Lannon think they can offer a scientific answer to that question. All three are M.D.'s and academic psychiatrists. They think the melancholy of contemporary society originates in the limbic brain, the part that controls emotions, affection, and play. The limbic brain gets too little attention in modern life, while the neocortex, the part that controls abstract thought, gets too much. We need more human contact, more emotion, and more play, they think -- for reasons of cell structure, brain chemistry, and evolutionary biology as much as for sheer satisfaction. Most specifically, they think human beings were designed to love, and in modern life aren't doing that enough.

An overlooked 2000 book by the three, "A General Theory of Love," lays out the scientific evidence that our hearts need as much attention as our heads. A General Theory of Love isn't a cute how-to or the kind of Seven Simple Steps to Finding True Love Effortlessly in 30 Seconds volume that lands authors on midmorning talk shows. It is a carefully reasoned book, dealing mainly with the findings of serious research. But it is also heartfelt and important, a powerful work. "A General Theory of Love" goes a long way toward explaining why people can become better-off, safer, and healthier, yet less happy -- and what can be done to change that.

First, according to the authors -- all of whom teach at the University of California at San Francisco School of Medicine -- is the matter of how the understanding of emotion has developed scientifically. In 450 B.C.E., Hippocrates proposed that emotions emanate from the brain. That, the authors contend, was about all science had to say about the subject for the next 2,000 years. Descartes would declare self-awareness the essence of human existence, but his focus was on abstract thought rather than emotion. When Darwin was formulating his theory of natural selection, he was nagged by the thought that emotions must have evolved, too. His final work, published in 1872, proposed that emotions developed because they conferred a survival advantage.

This last idea from the greatest figure in biology was greeted coldly by successive generations of researchers, who preferred to view human feelings as trite weepiness: Fear might have evolved so that animals would flee danger, but emotions such as love or fellowship were irrelevant to survival and thus of no scientific interest. Freud tried to write off emotions entirely, as hooey stemming from repressed childhood sexuality.

Following Freud, the behaviorists who dominated several decades of psychology asserted that everything was a trained response. By roughly the 1960s, the scientific consensus held emotions to be trivial distractions, weaknesses in human design. The ideal person, according to the scientific consensus of the time, would be "Star Trek"'s Mr. Spock, intelligent and honorable but completely freed of emotion.

But then, say the authors of "A General Theory of Love," came serendipitous discoveries. Drugs developed to combat tuberculosis were observed to improve patients' moods; later, antidepressants were discovered, essentially by accident. "How does one square that," the authors ask, "with the supposed preeminence of repressed sexual urges as the cause of all matters emotional?" Researchers began to understand that parts of the human emotional structure are biological in origin, influenced by the chemistry of the brain.

While this may sound at first glance like depressing determinism ("chemicals make us love," etc.), to many attempting to fathom human psychology it was a liberating thought. If emotions are biologically seated within the brain, then they are real and central to human life, not just some weepy distraction. Moreover, if emotions partly arise from our biology, then emotions must have evolved -- just as Darwin guessed -- and if they evolved that means they must benefit us or they wouldn't have been favored by natural selection.

The idea that emotions are here because they are beneficial was a breakthrough for humanistic psychology and currently constitutes one of the liveliest areas for research.

National Institutes of Health researcher Esther Sternberg recently devoted an excellent book to the hard-science evidence that positive emotions help regulate immune response, meaning good relationships with others literally improve health.