There have been several studies in the past little while done on this subject. Two that are often quoted are a German study from 2012 and an Australian one from 2014. There might be others; I'm not sure. The above-mentioned studies do show that there is a stronger incidence of some psychological problems, including depression and anxiety, among vegetarians and vegans.
Before you carnivores start doing a victory dance over the carcass of the poor murdered animal you're about to eat, consider the following points: Neither of the studies determined that a meat-free diet CAUSED the above-mentioned problems, only that there was a higher incidence of these problems in those who didn't eat meat. In fact, the German study concluded that "there was no evidence for a causal role of (a) vegetarian diet in the etiology of mental disorders." This study went on to state that many if not most of the participants had been diagnosed with said conditions BEFORE they switched to a meat-free diet. Does this conclusion mean that vegetarians are at least more prone to certain psychological issues? Possibly, but not because of our diet. Many people stop eating meat because they are sensitive, compassionate individuals. Is it really so strange that a sensitive person in a desensitized world would be more prone to anxiety, depression, etc.? Not really. Is it really so strange that a person who empathizes with the plight of animals could eventually break down when faced with the overwhelming lack of empathy from all around? Not really. It is easier to go through life with thick skin. I don't have scientific proof that people who don't let the evils of the world get to them too much are less prone to the above-mentioned ailments, but this would seem logical. But what about the folks who go vegetarian for health reasons? In relation to the study above, some of them might have done so because of an exaggerated concern about their health, something that could easily stem from a pre-existing psychological issue as well.
The truth of the matter is, there is no direct proof that not eating meat makes one more susceptible to developing psychological disorders. There is no proof because there is no direct causal relationship between the two. Some of my friends know that I had problems with anxiety a couple of years ago. Some might have even thought that this was because of my meat-free diet. Well, it wasn't. What most of my friends don't know is that I had the same problems in my mid-20s, back when I ate meat regularly.
To be fair, I'm not arguing that all vegetarians and vegans are perfectly healthy individuals. In fact, it is sometimes a challenge for some of us to obtain nutrients that are easily available in meat (things such as iron, B12, etc.), especially when we first make the switch. If someone is not careful, it is possible that a prolonged absence of certain nutrients might lead to physical and/or psychological problems. Thankfully, we live in times when a simple internet search will give you quick and easy ways to make sure you're getting all the nutrients you need. My B12 levels are through the roof, as are my iron levels (too high, in fact). I know a couple of people whose iron levels INCREASED when they went vegan. Whether you eat meat or not, a balanced diet is key, and a little bit of research goes a long way to making sure that your diet is balanced enough to ensure you're healthy.
