Forty years ago the economists Finn Kydland and Ed Prescott wrote a paper (for which they later won the Nobel Prize) observing that there are situations when the government makes a promise it can’t be expected to keep, and that policy inevitably reflects that reality.

For instance, they observed, the best policy regarding floods would be to simply disallow all construction in a floodplain, or at least forswear any public financial assistance to people who do build there and subsequently have property damaged by a flood.