It’s the start of a new year, so think ahead, if you will, to December 31, 2008. What are your hopes for the next 12 months? Maybe you want to be richer or slimmer, get married or get divorced, become gainfully employed or be thankfully retired.

There is a single word that describes the goal of all these dreams and aspirations. They are all ways, ultimately, to make you happy.

Some of us will get the things we want, and others won’t. The more interesting question is: Why do people who get what they want rarely end up as happy as they expected, while people who fail to achieve dreams rarely end up as unhappy as they feared?

A Harvard psychologist, Daniel Gilbert, has made it his life’s work to understand why people not only make errors in predicting what will make them happy, but also why they make the same errors over and over again.

Mr. Gilbert argues that the phenomenon has much in common with the way parents relate to their children. After decades of slogging and sacrifice, parents are shocked when children turn around and say, “Whatever made you think doing all this would make me happy?” In much the same way, Mr. Gilbert has argued in a number of research papers and his 2006 book, “Stumbling on Happiness,” that our future selves often have trouble understanding the choices and decisions we make today.

Mr. Gilbert argues that our inability to make accurate predictions about what will make us happy stems from thought processes that people are more or less stuck with — our minds are designed to see the world as it is right now, rather than from the point of view of the people we are going to become. Understanding this won’t make people happier, he argues, but it might help them understand themselves and their choices better.

— Rule no. 1: Bingeing is bad, except when it isn’t. Experiments have confirmed that, for example, two gifts of $50 make people happier than a single $100 gift. “The first million you earn means more to you than the second,” said Carey Morewedge, who studies social and decision sciences at Carnegie Mellon University. “After a certain point, you become insensitive to gains of the same size.”

But in a new paper, Mr. Morewedge published in the Journal of Experimental Psychology, he showed this intuition breaks down when the goodies in question are too small. People often go to two moderately priced restaurants instead of one really nice restaurant, and find that neither meal pleases them.

There is a minimum amount of pleasure, in other words, that must be achieved before people derive any satisfaction at all. Different people have different thresholds, but subdividing your pleasures below that threshold will result in less happiness, not more.

— Rule no. 2: Happiness often comes from what you don’t know. People generally dislike uncertainty and often go out of their way to reduce uncertainty. But in a series of experiments published in the Journal of Personality and Social Psychology in 2005, Mr. Gilbert and his colleagues showed that people who received gifts for no apparent reason felt happier than those who received identical gifts for reasons that were clear. “When we have uncertainty about the nature or cause or meaning of any event, it amplifies the emotional consequences of that event,” Mr. Gilbert said in an interview. “When you don’t understand why a bad thing happened, it is worse. When you don’t understand why a good thing happened, it’s better.”

While uncertainty about negative and dangerous things is unpleasant, the psychologists argued that we foolishly seek to apply the same principle to things that we know are positive.

— Rule no. 3: Keeping your options open won’t necessarily make you happier. Given the choice, people like to keep their options open.

When researchers asked people whether they preferred to take home a poster they had to keep or take home one that could be exchanged later on, most people chose the latter. But it was people who made irrevocable choices early on who ended up happier with their posters.

Mr. Gilbert said the finding prompted him to go home and propose to the woman he had been living with: “I always thought love causes marriage, but my data said marriage causes love,” he said. “When you lock yourself in something you cannot get out of, you will find ways to be happier. … I do love my wife more than I loved my girlfriend, and they are the same person.”

— Rule no. 4: The things you fear are not as bad as you think. Mr. Gilbert said a number of experiments have found that people overestimate how unhappy they will be after a tragic event, and this keeps them from taking risks. Torn between life choices? The experimental results suggest the worst option is usually indecision — no matter what choice people make, they are more likely to be okay with the consequences than if they stay on the fence.

Why do we systematically fail to predict how happy and unhappy we will be? For one thing, predicting the future is inherently difficult.

But even when we know what is going to happen, we base our estimates of our future happiness on the people we are today, and fail to appreciate not only that we will be different tomorrow, but that the very things we seek will change who we are.