Most mental disorders affect only a small segment of the population. On the reasonable assumption that minds or brains are prone to occasional malfunction, these disorders do not seem to pose distinctive explanatory problems. Depression, however, because it is so prevalent and costly, poses a conundrum that some try to explain by characterizing it as an adaptation—a trait that exists because it performed fitness-enhancing functions in ancestral populations. Heretofore, proposed evolutionary explanations of depression did not focus on thought processes; instead, (...) they emphasized that it facilitates navigation of adverse social circumstances or promotes immune response to infectious agents. According to a new hypothesis, the “analytical rumination hypothesis” (ARH), however, depression’s crucial adaptive trait is rumination—negative, intrusive thought. ARH holds that, (i) social dilemmas trigger depressed mood; (ii) depressed mood induces changes in body systems that facilitate ruminative analysis aimed at solving dilemmas; and, (iii) depressive rumination is a fitness-enhancing trait that was selected for in evolutionary time. Jointly, (i)~(iii) imply that we should not think of rumination as a disorder; instead, it is a trade-off, an eminently rational one. In the same way that fever solves a problem—coordination of the immune system in response to infection—so too does depressive rumination solve a problem, a social dilemma, albeit at the cost of inducing anhedonia and other maladies. But they argue that the cost is worthwhile, something that should be endured “until the problem is solved.” First, we argue that there are two distinct types of rumination, brooding and pondering; the former is associated with a disposition for depression, not the latter. But only the latter has the problem-solving capabilities that ARH requires. Second, recent brain imaging studies of depression reveal resting state hypoactivity in lateral regions and hyperactivity in paralimbic regions; this asymmetric pattern correlates with heightened levels of brooding, self-focused rumination. In other words, on the personal level, patients are trapped within self, isolated from the external world and suffused with negative affect; on the subpersonal level, this pattern is reflected by an asymmetric pattern of lateral vs. paralimbic resting state activity. Third, we proceed to conjecture that rational responses (e.g., pondering) to social dilemmas are those that strike a balance between internal and external considerations in the process of belief formation. Fourth, because the asymmetric resting state activity blocks those who suffer with depression from accessing and processing potentially positive stimuli from the external world, the capacity for rational, analytic response—hence, problem-solving—is constrained. Fifth, it follows that, although there might be conditions for which suffering should be endured rather than pharmacologically alleviated, depression is not one of those. Indeed, in view of the effects of the asymmetric resting state pattern, it is unlikely that depressive rumination would have been useful even for ancestral populations. (shrink)