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.
Is it possible that humans are complex, nuanced creatures with a variety of emotions and motivations that make sweeping generalizations appear comically simplistic? Nah. Let's just call the remaining 10% "spicy" and move on.