[throw0101c]

In the US at least, parties started sorting their policies on certain issues in the 1960s:

* https://en.wikipedia.org/wiki/Southern_strategy

Ezra Klein goes into the (US) history of this in his book:

* https://en.wikipedia.org/wiki/Why_We%27re_Polarized

It also has a few chapters on how humans seem to have built-in tribe/clan mechanism (us/them, in/out-group).