A Thrive/Survive Theory of the Political Spectrum

I admitted in my last post on Reaction that I devoted insufficient space to the question of why society does seem to be drifting gradually leftward. And I now realize that in order to critique the Reactionary worldview effectively we're going to have to go there.

The easiest answer would be "because we retroactively define leftism as the direction that society went". But this is not true. Communism is very leftist, but society eventually decided not to go that way. It seems fair to say that there are certain areas where society did not go to the left, like in the growth of free trade and the gradual lowering of tax rates, but upon realizing this we don't feel the slightest urge to redefine "low tax rates" as leftist.

So what is leftism? For that matter, what is rightism?

Any theory of these two ideas would have to explain at least the following data points:

1) Why do both ideologies combine seemingly unrelated political ideas? For example, why do people who want laissez-faire free trade empirically also prefer a strong military and oppose gay marriage? Why do people who want to help the environment also support feminism and dislike school vouchers?

2) Why do the two ideologies seem broadly stable across different times and cultures, such that it's relatively easy to point out the Tories as further right than the Whigs, or ancient Athens as further left than ancient Sparta? For that matter, why do they seem to correspond to certain neural patterns in the brain, such that neurologists can determine your political beliefs with 83% accuracy by examining brain structure alone?

Link: https://slatestarcodex.com/2013/03/04/a-thrivesurvive-theory-of-the-political-spectrum/