Quoted from PCRM:
Vegetarians have lower instances of depression, according to a new study in Nutrition Journal. Researchers looked at 60 vegetarians and 78 meat-eaters in the southwestern United States and found that vegetarians scored significantly better on standardized mood tests. The mood tests measured depression, anxiety, and stress and were compared to food frequency questionnaires. The vegetarians consumed less eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid—all animal sources of omega-6 fatty acids—but reported higher mean intakes of plant sources of omega-6 and omega-3.
Who knows whether these claims are strictly accurate - I so often find there's some sort of bias involved or that it's not statistically viable so I'm a bit sceptical of these things now.
But if it is true WOOHOO! (that's my vegan good cheer speaking)