via Matt Yglesias:
Suppose the country is forced to choose between two courses of action, A and B. A is guaranteed to result in situation A* which is of quality 2. B has a 90 percent chance of resulting in situation B* which is of quality 10, but has a 10 percent chance of resulting in situation B$ which is of quality -100. A-advocates will point out that their favored course of action has a higher expected return. B-advocates will say that the overwhelming preponderance of the evidence supports the conclusion that B will have better results. So what happens if the B-advocates win? Well, we adopt policy B and probably move to situation B*. At this point the A-advocates become discredited in the eyes of the media and the sheep-like public who dismiss them as a band of chicken littles. Then the situation iterates and we choose B again. Same result. And again, and again, etc. until soon enough we wind up in B$ where all our gains have been wiped out and then some.