Politicians break their promises because voters don’t rationally punish political promise-breaking.  But why do politicians want to break their promises in the first place?  What’s the point of promising X, then backing out?

The simplest answer is just that circumstances change between promise time and action time.  Example: The politician says, “I’ll keep us out of the war,” then the country gets sneak attacked.   He doesn’t keep his promise because people no longer want him to.

Another mechanism: Some people pay more attention to politics during campaigns, others during time in office.  For example, regular voters pay a lot more attention to trade policy during election season that at other times; the for exporters, pre- and post-election attention ratios go the other way.  So as long as voters don’t compensate with probability multipliers, it makes sense for candidates to loudly oppose NAFTA during the election, then quietly drop the issue.

In my view, though, the most interesting and important reason why politicians break their promises is that voters have misconceptions about the effects of their favorite policies.  The upshot: If politicians did exactly what voters say they want, the results would be bad, and the politician would get blamed.  Under the circumstances, politicians who want to get elected promise to do as the people command, then “betray” them for their own good.  Making the promises helps politicians attract popular support before they get in office.  Breaking the promises helps politicians avoid losing popular support after they get in office.

A old saying tells us, “Thank goodness we don’t get as much government as we pay for.”  I’m tempted to add, “Yes, and thank goodness politicians don’t actually do exactly what they promised.”  Dishonest politics is sordid, but honest politics is absolutely scary.