It seems that only some types of people would be willing to make the decision to go vegetarian. Have there been personality studies of people who are most and least likely to go vegetarian, and how are they different from the general population? Do different personalities become vegetarian in different cultures?
This is clearly hard to answer, as it is very broad, but there are relevant sources on the matter.
- Intelligence - vegetarians may be more intelligent than average - according to this study that examined the relation between IQ in childhood and vegetarianism in adulthood:
Higher scores for IQ in childhood are associated with an increased likelihood of being a vegetarian as an adult.
This source provides more insight into vegetarians profile.
- Better education and higher occupation
most vegetarians have professional or managerial positions
Mostly women (more than 70%). This might be related to the fact that women are more empathetic than men. (this source argues about being vegetarian and being empathetic correlation). Also, according to this source, "it has been suggested that meat consumption makes men feel more masculine, but it remains unclear whether this is the case and whether it is affected by social context".
Vegetarianism does not appear to be a fad. In fact, eight years was the median length of time that study participants had been vegetarians.
For most vegetarians, vegetarianism is more than what not to eat: It's an ideology of how life should be lived.
- Health conscious
In a population-based study in British Columbia (BC), the survey results show vegetarians appear to be more health conscious than non-vegetarians.
As a side note, there is a very interesting article for the opposite of what is being asked - Psychology of eating meat
As a conclusion, I would say that a random vegetarian/vegan living in Western world will most likely be/have more than average: a higher intelligence, better education, higher occupation, be a woman, health conscious.