When positive psychology began, it was all about helping "the other 80%"—those who don't have a major problem to fix, and who therefore receive very little attention in the research literature. At the time, Seligman (the father of the positive psychology movement) had suggested that there should be a science of becoming happier to mirror the science of treating mental disorders. It was because of this very suggestion that began my research career. It's a worthy goal, and in the past 10+ years, there has been a lot of progress towards learning how people can improve their own happiness.
However, there hasn't been much basic research just trying to learn what the "other 80%" is like. This is quite a contrast to clinical psychology, where research on how to treat a disorder is paralleled by research on who tends to get the disorder and why. In clinical psychology, basic and applied research interact to produce more solid science. This happens less in the study of happiness interventions; researchers have launched into a full-fledged investigation of how to make "the other 80%" happier, but we don't know where they are starting off!
To begin to answer this question, I did a study, which I recently published with Matt Della Porta, Russell Pierce, Ran Zilca, and Sonja Lyubomirsky in Emotion (Study 1). We looked at a large dataset comprised of people who were perusing Martin Seligman's self-help website, Authentic Happiness; we had a link that said "Participate in Positive Psychology Research" on the side of the page, and people who followed that link found our study. We hoped that this sample would be representative of the types of people who are interested in becoming happier—at least, more representative than the typical study population, which consists of college students.