What we're learning, these thinkers argue, should make us reconsider some of the basic rules by which government regulates behavior: how we litigate lawsuits and write contracts; how we zone neighborhoods; which medical research we fund, and how we prioritize healthcare. The findings of happiness researchers offer a new and potentially powerful set of tools to compare the impacts of various laws: how does it change everybody's happiness if the minimum wage is raised, if the speed limit is lowered, if divorce laws are loosened?

These findings have fed the growth of a burgeoning "positive psychology" movement focused on helping people enrich their own lives. But now some scholars are starting to ask a bigger question: shouldn't this new understanding affect policy, too? A huge range of social systems, from tort law to urban planning to medical care, are built on assumptions about what makes people happy. Now, for the first time, researchers are claiming to be actually measuring happiness, to actually know what causes it. In a society whose founding document asserts a basic right to the pursuit of happiness, that new knowledge could have far-reaching implications.

In recent years, cognitive scientists have turned in increasing numbers to the study of human happiness, and one of their central findings is that we are not very good at predicting how happy or unhappy something will make us. Given time, survivors of tragedies and traumas report themselves nearly as happy as they were before, and people who win the lottery or achieve lifelong dreams don't see any long-term increase in happiness. By contrast, annoyances like noise or chronic pain bring down our happiness more than you'd think, and having friends or an extra hour of sleep every night can raise it dramatically.

But in fact, it's back pain that causes more misery. Most blind people would like to be able to see, of course, but once they've figured out how to live a sightless life, their blindness doesn't really make them unhappy. Chronic pain, on the other hand, sours our mood with every new twinge, and we never really adapt to it.

IF YOU WERE given the choice, and you wanted to reduce human suffering by as much as possible, would you cure blindness or back pain? It seems a silly question. The thought of losing one's sight is, to most people, as frightening as it is depressing: we would no longer be stirred by sunsets or landscapes or the expressions on the faces of our loved ones. Everyday chores would become more difficult, crossing the street perilous. Many sports and pastimes would simply be off-limits, and we would lose a good deal of our independence.

Because serious happiness research is still new, these proposals remain broad, even speculative. But underlying all of them is the likelihood that if our longtime assumptions about happiness are wrong, they have led to ineffective, even counterproductive, laws and policies.

"This work challenges not only the way the legal system works, but a lot of the assumptions that we've been making about the way people act and think and behave and feel," says Jeremy Blumenthal, an associate professor at the Syracuse University College of Law.

Some skeptics think it's premature to let happiness research dictate policy: much of the data is still provisional, and some findings seem to contradict each other. And, more fundamentally, some argue that no amount of data could justify the sort of paternalism that would be required for the government to force people into some happiness-maximizing choices. It's part of a broader debate that has preoccupied thinkers since the dawn of philosophy but been rekindled by the new research: how do we define happiness, anyway, and how much should we value it?

The patron saint of the happiness maximizers is Jeremy Bentham, the English philosopher who two centuries ago gave the world the ethical theory known as utilitarianism. The theory itself is simple: in any situation, the best thing to do is that which brings the greatest aggregate pleasure or happiness. Bentham imagined a "hedonic calculus": a systematic reckoning that weighed factors like intensity of feeling, duration, purity and the number of people affected. One of his disciples, Francis Edgeworth, wrote hopefully of a future when utilitarians could use a device he called a "hedonimeter" to simply read out a person's happiness level.

Since then, social scientists and policymakers have tended to reject Bentham's ideas as unusably vague. The "hedonic calculus" seemed too riddled with subjectivity to base real-world social policies on.

But some psychologists have begun to argue that you can, in fact, reliably measure happiness: all you need to do is ask people. Because we so badly mispredict and misremember how happy something made us, however, the trick is to ask people to rate their current happiness, and then track the changes over time. Many recent studies have subjects keep happiness diaries; others give them beepers and have them rate their happiness whenever beeped. These studies, matched up with research that relies on more general happiness self-reporting, have begun to provide a consistent, and occasionally surprising, portrait of what really feeds and impedes happiness.