Psychologist Richard Davidson and the Dalai Lama did some research on it a couple years back. As I understand, the Dalai Lama had heard about Davidson's work on hemispheric dominance and personality dispositions. He had found that people with left hemispheric dominance were happier than people who were dominant on the right. Most people assumed that you couldn't change stuff like that. The Dalai Lama thought that his monks who were trained in meditation were all happy and that if you trained people in mediation, they would show more activity on the left. The had people who were known to be right dominant and had them trained to meditate by monks. They then had their brains scanned. They found that participants showed a shift to more left processing. So I think there is some scientific evidence that meditation can have long term positive effects.