The envious group was the most common, accounting for 30% of participants, although it's possible these volunteers were just jealous of everyone else who didn't have to fill out dozens of test-like questions. The other groups had 20%, and a fifth group made up 10% -- scientists leading the study admit that this smaller group defies a one-word description, goshdarnit, and they can't seem to figure it out. So there's 10% of the population running around being unpredictable and uncategorized, and frankly they're probably to blame for all of society's ills.
The pessimistic group was described as making decisions based on what the lesser of two evils is, while optimists want to make the best decision for everyone. Sometimes those are the same thing! Trusting people put faith in a partner's willingness to act on their behalf, while envious people don't really care about the outcome, as long as it's better than other people's.
Basically, the upshot is that human beings aren't nearly as complicated as we're made out to be; some people are just born collaborators, while others would prefer to do things by themselves, no matter how destructive. So, rather than wasting time dissecting someone's personality traits, just toss everyone into one of these buckets and go about your business.