ne of the most often-repeated parts of the Declaration of Independence may also be one of the least influential. People march in Washington protesting various assaults against the first two unalienable rights, life and liberty. The third, however, rarely merits mention beyond the simple recitation of the Declaration's most famous sentence.

Instead, the pursuit of happiness is left to the free market. Americans are typically responsible for their own emotional well-being, and, according to economic theory at least, there has never been much mystery about how to achieve it. Textbooks call this well-being "utility," and they say it rises alongside a person's wealth. "If there is a more fundamental idea in economics, I'm not sure what it would be," said Andrew J. Oswald, who teaches the subject at the University of Warwick in England.

There is a fundamental problem with the idea, however. Over the last half-century, the United States has become a far richer country, with ordinary people able to afford items — a second car, a transcontinental plane ticket, a machine to show movies at home — that only the wealthy owned before World War II. On average, people today can buy better food, better health care and seemingly a better life.

Yet the nation as a whole describes itself as no happier. In fact, over the last 30 years, Americans have become somewhat less satisfied with their lives, according to an array of surveys. The obvious conclusion is that an everyday cliché may in fact have a more accurate take on modern life than Economics 101: money really cannot buy happiness.

That hardly passes for credible social science, though. And so a growing group of economists are now shedding the profession's traditional skepticism about personal interviews and are scouring survey data in an effort to explain the contradiction. In doing so, they have rejected their discipline's circular view of happiness, that people choose to do what makes them happy and what makes them happy is what they choose to do. Thus, asking them whether they are happy is unnecessary.

The happiness researchers are joining psychologists and sociologists who have been studying the issue for years. Last fall, for example, the Woodrow Wilson School of Public and International Affairs at Princeton opened a Center for Health and Well- Being. Nearly half of the professors associated with the center, including its director, are economists.

One of the field's most intriguing early conclusions holds that money does indeed make people happier but that it is less potent than imagined. When people inherit a large sum of money, for instance, they become more satisfied with their lives, according to recent research. But over the last 60 years, and particularly the last 30, a powerful set of social forces has outweighed the effect that rising incomes have had on people's well- being. People work more hours, lose their jobs more often and, most importantly, get married less and divorced more than they did in the past.

All that helps explain why the average American family could have received a 16 percent raise between 1970 and 1999, while the percentage of people who described themselves as "very happy" fell from 36 percent to 29 percent. "Money does buy happiness," said David G. Blanchflower, an economist at Dartmouth, who with Mr.. Oswald has studied well- being in England and the United States. "It just hasn't bought enough."

Statisticians call this Simpson's paradox. In England, for example, married people are as a group happier than they were three decades ago. Unmarried people are, too. But because the ranks of the unmarried have grown and because unmarried people are not as satisfied as married people, overall happiness has still declined.

In a recent paper, Mr. Blanchflower and Mr. Oswald even put a crude cost on certain life conditions. A lasting marriage seems to be worth $100,000 a year, they said, because all else being equal, a married person is as happy as a divorced- and-not-remarried person who makes $100,000 more than the married person. Losing a job and remaining unemployed costs men $60,000 worth of happiness a year. And in what the authors described as yet more evidence of discrimination, being black costs $30,000 a year of happiness.

The research has also found that women remain happier than men today, but that the gap has narrowed as — and perhaps because — women have entered the work force in larger numbers. In fact, men are about as happy today as they were in the early 1970's; nearly all of the decline since then is among women.

The data have even offered a bit of an explanation for the dread with which many people view their 40th birthdays. It turns out that happiness reaches a nadir for most people right around 40. Before then, people seem to focus on all of the possibilities in front of them. After 40, more seem to be satisfied with what their life entails at the moment, Mr. Blanchflower and Mr. Oswald say.

(In contrast, two psychologists who have studied happiness — Ed Diener and David G. Myers — have emphasized in their work that happiness is actually very similar across genders, ages and races.)

Of course, much of the research is based on self-evaluation. People are considered satisfied if they say they are satisfied, and their analysis can change if their expectations grow or the meaning of the word "happy" subtly shifts. This is one reason many economists have long avoided such research and have instead studied subjects like income, which have a solid quantitative underpinning. Even psychologists, who are more comfortable with survey data, say that the lack of a fixed reference point creates problems.

"It is possible that we're somewhat happier," said Daniel Kahneman, a psychologist at Princeton who sits on the executive committee of the new well-being center, "but because we demand more of life, we don't know we're happier."

"The evidence isn't in," he added.

Still, there is a long line of research that has connected self-reported happiness to more objective conditions, according to a paper by James D. Konow and Joseph E. Earley of Loyola Marymount University in Los Angeles. People who say they are happy tend to have low blood pressure. They more often make a genuine smile (known scientifically as a Duchenne smile, when two different facial muscles fire). And their spouses and friends often describe them as happy.

Happiness researchers say their biggest problem is that data remain somewhat flimsy. Most surveys on life satisfaction take a snapshot of a population at a given moment rather than conducting longitudinal studies following people over the course of their lives. These longitudinal studies, like one started in Britain in 1989, allow researchers to make better guesses about what causes unhappiness rather than simply what conditions unhappiness is correlated with.

Take marriage. It could cause people to become happier by giving them companionship. But it is also possible that happy people are more likely to get married because they have an easier time finding a mate or because they are more willing to believe a relationship could last for a half-century. By following a specific group of people throughout their lives rather than interviewing a new sample every year, researchers can more accurately analyze when people's emotional well-being changes.

Though the longitudinal studies are expensive, they are becoming more popular. "There's a whole new field emerging," Mr. Oswald said. "We're right at the frontier."

The current happiness movement has its roots in a 1974 paper by Richard Easterlin that researchers still mention frequently. Mr. Easterlin, then a professor at the University of Pennsylvania, pointed out the same paradox that is keeping economists busy today: even though income had risen dramatically since World War II, Americans said they were no happier.

In the paper, he said people's expectations were a major reason. Once people acquire something they want, their goals change, and they desire something new. Society lives on a "hedonic treadmill," he said, using a phrase coined by two other researchers.

Mr. Easterlin also drew on the work of James Dusenberry, a fellow economist who said people care more about their income relative to their peers' than they do about absolute income. In recent years, two economists whose work has made it into the mainstream — Robert H. Frank, the author of "Luxury Fever," and Juliet B. Schor, who wrote "The Overspent American" — have made similar arguments.

The idea is not necessarily irrational, Mr. Frank says. A family could be able to afford more amenities than its predecessors in an earlier generation, but it might find itself unable to buy a home in a neighborhood with good schools if it does not rate high enough in today's income distribution.

This argument can also offer a fairly benign explanation for the recent decline in happiness, precisely because it has occurred among women. As women have entered the work force in greater numbers, their expectations — for what kind of job they have or how much money they make — could have risen more quickly than the quality of their lives.

Mr. Blanchflower is skeptical, though. He notes that reported happiness among black Americans has risen over the same time period and that they would seem to be in a similar situation as women. A more likely explanation — although one yet to be supported by data — is that women have become less satisfied because many of them now work but face responsibilities at home similar to those their mothers did, he said.

In fact, some researchers think the faster pace of life today, as compared with that of recent decades, is a possible cause of the disconnection between income and happiness. Using data from the International Social Survey Program, a research consortium, Mr. Blanchflower and Mr. Oswald found that an unusually large percentage of Americans said they wanted to spend more time with their families.

In Japan, 44 percent of people polled fit into this category. In Denmark, Israel and Italy, about 65 percent did. In western Germany and Britain, about 75 percent did. And in the United States, 85 percent said they wished they could be with their families more.

In theory, many Americans today are financially secure enough to make this happen. They could simply work fewer hours and still have enough money to afford not only food and shelter but also a good bit of entertainment.

All of this is speculation, researchers acknowledge. Even 27 years after Mr. Easterlin noticed the divergence between wealth and happiness, there are no overarching explanations that are firmly grounded in data. Even given the rise in divorce and hours worked, for example, economists say they would have expected income gains to have made people more satisfied than they are.

"We're looking for a big idea, a big breakthrough," Mr. Blanchflower said. "This is hugely important, and we don't know the answer."