Psychologists have long wondered why some people have a sunny disposition and others are grouchy. One school of thought holds that people are just born that way and that these personality traits don't change much. People bounce back to their natural happiness level after adverse as well as exhilarating life events.

But new research from Michigan State University's Richard Lucas indicates that some life events do have the potential to change us and make us happier -- or unhappier -- permanently.

Lucas and his colleagues analyzed large surveys from Germany and Great Britain started by economists in the 1980s. Over the years, the surveys asked tens of thousands of people about their level of satisfaction with their lives. They also gathered information about the subjects' life events. "So for example, we can look at people who start out married, get divorced at some point and stay divorced," Lucas says, "and we can compare their happiness before and after the divorce to see whether that life event is associated with any lasting changes. This tells us, using pretty strong methodology, whether or not life events are associated with any permanent changes in happiness."

The honeymoon glow that comes with marriage doesn't last

Lucas looked at events such as marriage, divorce, losing a spouse or becoming disabled. "We found for the one positive event that we were looking at, marriage, there we did find adaptation; within two years, people adapted back to their original level of happiness." In other words, the honeymoon glow eventually mellows.

But, Lucas says, negative events tend to produce a more permanent change, and people become unhappier and less satisfied with their lives.

Negative events, like divorce, tend to have a permanent impact, making us less happy the rest of our lives

"So people who experience the divorce seem to be lower in their level of happiness after the divorce than they were before the divorce, and people who became disabled were much lower after their disability than they were before the disability." Those who were widowed fared a bit better, the researcher says, 'but it took a long time - about 7 years - for them to return to [the happiness level] where they were before they lost their spouse." However, Lucas says these statistics don't account for individual differences in how people react to life events.

Lucas says one possible avenue for further research could be to identify events that tend to make people happier, permanently. The research appears in the April issue of Current Directions in Psychology.