Why rival candidates' platforms end up looking the same

Duncan Black proved that when voters' preferences fall along a single left-right spectrum, the option preferred by the voter exactly in the middle beats every other option under majority rule. That result explains why competing politicians, each just trying to win, keep drifting toward the same centrist position.

— Duncan Black, On the Rationale of Group Decision-Making — Journal of Political Economy, Vol. 56, No. 1, 1948