John Urschel was the only player in the N.F.L. simultaneously getting a math Ph.D. at M.I.T. But after a new study came out linking football to brain damage, he abruptly retired. Here's the inside story — and a look at how we make decisions in the face of risk versus uncertainty.

How do so many ineffective and even dangerous drugs make it to market? One reason is that clinical trials are often run on "dream patients" who aren't representative of a larger population. On the other hand, sometimes the only thing worse than being excluded from a drug trial is being included.