There is a vigorous academic debate over whether negative advertising depresses or increases voter turnout. I suspect it does both, depressing turnout among moderates and independents while stimulating it at the ideological extremes. In that process, what has changed is the composition of the turnout rather than its overall level…

The irony is that the politicians who prevail in these gladiator contests inherit a system so bitter, so partisan and so ideologically polarized that they can’t accomplish anything. They know that they and their constituents would be better off if they cooperated and compromised more, but they just can’t. If they try, they face a serious risk of being run out of office, either in the next primary by someone who better appeals to the party’s political base, or in the general election by an opponent whose extremism has allowed him or her to energize the other side’s core voters.

Politics has become a tragedy — a tragedy of the commons, that is. The individual pursuit of rational self-interest by parties and politicians, which in political and economic theory is supposed to generate the best outcome, has instead led to a cycle in which extremism, partisanship and stalemate all beget more of the same. We keep thinking it can’t continue like this, but it only gets worse.

Some may complain that this analysis falls into the trap of moral equivalency, failing to note that Republicans practice the politics of extremism and suppression of the moderate vote and that Democrats offer more moderation and compromise. But while it is true that the move away from the political center has been asymmetric and probably began with the Republicans, the success of that strategy has now forced everyone to play the same game.