In the following passage, why did the author use contributes, not contribute? Is the subject of the sentence the corresponding lack of social and relaxation time? Or, did the author regard this extra stress, and the corresponding lack of social and relaxation time as a single concept?
Once we recognize the falsecause issue, we see it everywhere. For example, a recent longterm study of University of Toronto medical students concluded that medical school class presidents lived an average of 2.4 years less than other medical school graduates. At first glance, this seemed to imply that being a medical school class president is bad for you. Does this mean that you should avoid being medical school class president at all costs? Probably not. Just because being class president is correlated with shorter life expectancy does not mean that it causes shorter life expectancy. In fact, it seems likely that the sort of person who becomes medical school class president is, on average, extremely hardworking, serious, and ambitious. Perhaps this extra stress, and the corresponding lack of social and relaxation time―rather than being class president per se―contributes to lower life expectancy. If so, the real lesson of the study is that we should all relax a little and not let our work take over our lives.