The researchers came to this conclusion after studying Seventh Day Adventists, who were split about 4-5 between vegetarians and meat-eaters. Rodale notes that this group was chosen because the nature of the community is such that it is easier to account for external influences than diet coming into play.

The researchers found the vegetarians reported diets significantly lower in EPA and DHA, the omega-3 fatty acids that we get from eating fish, and which many studies have found are a key factor in improving both physical and mental health. So they expected to find the vegetarians would have higher incidences of issues like depression, anxiety, and mood problems. Instead, they found the opposite result. Vegetarians scored lower on depression tests and had better mood profiles than their fish- and meat-eating peers.