[ojbyrne]

Social sciences generally can't manage the strict experimental methods we're taught in science so they rely on quasi-experimentation (and are fully aware that that is less rigorous). The control you suggest would be confounded by the problem of age. What if people get more (or less) creative just through the passage of time?

Not that I'm defending this study, though it may be extremely rigorous, it's really impossible to know from an economist article.