In the mathematics of social choice theory, there are a lot of unusual results about 2-party versus +2-party electoral systems.

Check out the Gibbard–Satterthwaite theorem:
https://en.wikipedia.org/wiki/Gibbard%E2%80%93Satterthwaite_theorem

And the more famous Arrow's impossibility theorem:
https://en.wikipedia.org/wiki/Arrow%27s_impossibility_theorem

I always cite these two theorems when people talk about the problems with 2-party electoral systems.