Now you have more than one reason to tell the world why your friend is one in a million - that is, if you are one of those who feel friendship is all about caring and sharing. A study suggests that being good to others adds meaning to your life, and makes you happy.

Michael Steger, a psychologist at the University of Louisville in Kentucky, has always been amazed by how differently people lead their lives.

Pat Tillman, for example, left the NFL to enlist in the Army and fight in Iraq and later Afghanistan (where he was killed), Steger said, but Paris Hilton continually pursues "a public life of shallowness."

Steger couldn't help but wonder which behaviour makes people happier - seeking pleasure or doing good?

To find out, he and his colleagues asked a group of 65 undergraduates to complete an online survey each day for three weeks that assessed how times they participated in hedonic, or pleasure-seeking behaviours, versus meaningful activities, such as helping others, listening to friends' problems and/or pursuing one's life goals.

The surveys asked the subjects how much purpose they felt their lives had each day and whether they felt happy or sad. The subjects also completed two sets of questionnaires at the beginning and end of the study to assess how they felt about their lives more generally.

They found that the more people participated in meaningful activities, the happier they were and the more purposeful their lives felt. Pleasure-seeking behaviours, on the other hand, did not make people happier. Realising that some people may feel guilty about reporting pleasure-seeking behaviours, Steger and his colleagues then modified the survey questions slightly to make them seem less exceptionable, and asked a new group of students to perform the study again, this time over a four-week period. The psychologists got the same results.

"A lot of times we think that happiness comes about because you get things for yourself," said Richard Ryan, a psychologist at the University of Rochester, who was not involved in the study. "But it turns out that in a paradoxical way, giving gets you more, and I think that's an important message in a culture that's pretty often getting messages to the opposite effect."

In order to make sure that the relationship between happiness and doing good wasn't the other way around - that happiness instead leads people to do good things - the researchers looked at which tended to come first. They found that the subjects became happier after they did something good, suggesting that happiness does, in fact, come about as a result of doing good things.

The results of the study, to be published in the Journal of Research in Personality, present an "enormously optimistic picture of people, that as a cynic, I was very happy to see," said Steger.