Dr. Robert Enright from the University of Wisconsin–Madison founded the International Forgiveness Institute and is considered the initiator of forgiveness studies. He developed a 20-Step Process Model of Forgiveness. Recent work has focused on what kind of person is more likely to be forgiving. A longitudinal study showed that people who were generally more neurotic, angry and hostile in life were less likely to forgive another person even after a long time had passed. Specifically, these people were more likely to still avoid their transgressor and want to enact revenge upon them four and a half years after the transgression.
Studies show that people who forgive are happier and healthier than those who hold resentments. The first study to look at how forgiveness improves physical health discovered that when people think about forgiving an offender it leads to improved functioning in their cardiovascular and nervous systems. Another study at the University of Wisconsin found the more forgiving people were, the less they suffered from a wide range of illnesses. The less forgiving people reported a greater number of health problems.
The research of Dr. Fred Luskin of Stanford University shows that forgiveness can be learned. In three separate studies, including one with Catholics and Protestants from Northern Ireland whose family members were murdered in the political violence, he found that people who are taught how to forgive become less angry, feel less hurt, are more optimistic, become more forgiving in a variety of situations, and become more compassionate and self-confident. His studies show a reduction in experience of stress, physical manifestations of stress, and an increase in vitality.
One study has shown that the positive benefit of forgiveness is similar whether it was based upon religious or secular counseling as opposed to a control group that received no forgiveness counseling.
The above is from Wikipedia.