The conclusion is based on two experiments. In the first, researchers had a group of men and women pray one single prayer for their romantic partner's well being. Others—the experimental controls—simply described their partner, speaking into a tape recorder.
The team then measured the participants forgiveness, which was defined as the diminishing of the initial negative feelings that arise when people have been wronged. Their results showed that those who had prayed for their partner harbored fewer vengeful thoughts and emotions.
In a second study, the researchers had a group of men and women pray for a close friend every day for four weeks. Others simply reflected on the relationship, thinking positive thoughts but not praying for their friend's well-being. They also added another dimension. They used a scale to measure selfless concern for others—not any particular person but other people generally. They speculated that prayer would increase selfless concern, which in turn would boost forgiveness.
So How does this common spiritual practice exert its healing effects? Researchers are unsure: most of the time, couples profess and believe in shared goals, but when they hit a rough patch, they often switch to adversarial goals like retribution and resentment. These adversarial goals shift cognitive focus to the self, and it can be tough to shake that self-focus. Prayer appears to shift attention from the self back to others, which allows the resentments to fade.
Citation: N. M. Lambert et al., 'Motivating Change in Relationships: Can Prayer Increase Forgiveness?', Psychological Science, December 2009; doi: 10.1177/0956797609355634