Imagine you have a choice between earning $50,000 a year while other people make $25,000 or earning $100,000 a year while other people get $250,000. Prices of goods and services are the same. Which would you prefer? Surprisingly, studies show that the majority of people select the first option. As H. L. Mencken is said to have quipped, "A wealthy man is one who earns $100 a year more than his wife's sister's husband."

This seemingly illogical preference is just one of the puzzles that science is trying to solve about why happiness can be so elusive in today's world. Several recent books by researchers address the topic, but my skeptic's eye found a historian's long-view analysis to be ultimately the most enlightening.

Consider a paradox outlined by London School of Economics economist Richard Layard in Happiness (Penguin, 2005), in which he shows that we are no happier even though average incomes have more than doubled since 1950 and "we have more food, more clothes, more cars, bigger houses, more central heating, more foreign holidays, a shorter working week, nicer work and, above all, better health." Once average annual income is above $20,000 a head, higher pay brings no greater happiness. Why? One, our genes account for roughly half of our predisposition to be happy or unhappy, and two, our wants are relative to what other people have, not to some absolute measure.

Happiness is better equated with satisfaction than pleasure, says Emory University psychiatrist Gregory Berns in Satisfaction (Henry Holt, 2005), because the pursuit of pleasure lands us on a never-ending hedonic treadmill that paradoxically leads to misery. "Satisfaction is an emotion that captures the uniquely human need to impart meaning to one's activities," Berns concludes. "While you might find pleasure by happenstance--winning the lottery, possessing the genes for a sunny temperament, or having the luck not to live in poverty--satisfaction can arise only by the conscious decision to do something. And this makes all the difference in the world, because it is only your own actions for which you may take responsibility and credit."

Harvard psychologist Daniel Gilbert goes deeper into our psyches in Stumbling on Happiness (Knopf, 2006), in which he claims, "The human being is the only animal that thinks about the future." Much of our happiness depends on projecting what will make us happy (instead of what actually does), and Gilbert shows that we are not very good at this forethought. Most of us imagine that variety is the spice of life, for example. But in an experiment in which subjects anticipated that they would prefer an assortment of snacks, when it actually came to eating the snacks week after week, subjects in the no-variety group said that they were more satisfied than the subjects in the variety group. "Wonderful things are especially wonderful the first time they happen," Gilbert explains, "but their wonderfulness wanes with repetition."

This habituation to even a multiplicity of wonderfulness is what economists call "declining marginal utility" and married couples call life. But if you think that an array of sexual partners adds to the spice of life, you are mistaken: according to an exhaustive study published in The Social Organization of Sexuality (University of Chicago Press, 1994), married people have more sex than singles--and more orgasms. Historian Jennifer Michael Hecht emphasized this point in The Happiness Myth (Harper, 2007). Her deep and thoughtful historical perspective demonstrates just how time- and culture-dependent is all this happiness research. As she writes, "The basic modern assumptions about how to be happy are nonsense." Take sex. "A century ago, an average man who had not had sex in three years might have felt proud of his health and forbearance, and a woman might have praised herself for the health and happiness benefits of ten years of abstinence."

Most happiness research is based on self-reported data, and Hecht's point is that people a century ago would most likely have answered questions on a happiness survey very differently than they do today.

To understand happiness, we need both history and science.