Peer review is often taken to be the main form of quality control on academic research. Usually journals carry this out. However, parts of maths and physics appear to have a parallel, crowd-sourced model of peer review, where articles are posted on the arXiv to be publicly discussed. In this article we argue that crowd-sourced peer review is likely to do better than journal-solicited peer review at sorting articles by quality. Our argument rests on two key claims. First, crowd-sourced peer review will lead on average to more reviewers per article than journal-solicited peer review. Second, due to the wisdom of the crowds, more reviewers will tend to make better judgements than fewer reviewers will. We make the second claim precise by looking at the Condorcet jury theorem as well as two related jury theorems developed specifically to apply to peer review.