Nope, it's the pluralist vote. Think about it - start with any number of parties > 2, and consider them as entities vying for resources (voters). The entity with the most resources wins. A resource is 'owned' completely by one entity only (one vote is all you get to cast). Entities can combine to pool resources.

Mathematical end result: 2 parties, split 50/50, desperately scrabbling for a thin margin in between, the swing voter.

Until pluralist voting is tossed out, this is what we're going to be stuck with.