Scott Sumner has some thought-provoking reactions to my critique of Scott Alexander’s Thrive/Survive Theory of left and right.  Here’s my reaction, point-by-point.  Scott’s in blockquotes; I’m not.

Liberalism is what happens when you are optimizing for a safe
environment, and illiberalism is what happens when you optimize for
thriving in an unsafe environment.

Now of course this raises a whole new set of issues. What do I mean
by ‘liberalism’ and ‘illiberalism’? When I say liberalism, I am
including classical liberalism, social democratic liberalism and
neoliberalism. I’m basically referring to utilitarianism. When I say
illiberal, I am referring to a wide variety of non-utilitarian views,
including class warfare (Mao), fascism (Hitler), white nationalism
(Bannon), racism (KKK), reverse racism (SJWs), tribalism (Afghanistan),
religious fanaticism, militarism, etc.

This is deeply puzzling. 

First, if “Liberalism is what happens when you are optimizing for a safe
environment, and illiberalism is what happens when you optimize for
thriving in an unsafe environment,” are we talking about selfish optimization or social optimization?  If the former, then how does being rich make caring about outsiders “selfishly optimal”?  If the latter, then it sounds like utilitarianism requires illiberalism in unsafe environments.

Second, classical liberalism, social democratic liberalism, and neoliberalism have all been widely accused of ignoring the interests of wide swaths of the population.  Classical liberals and neoliberals allegedly ignore the interests of the poor; social democrats allegedly ignore the interests of taxpayers and entrepreneurs. 

Third, most – perhaps all – of what Scott calls “illiberal” views have been defended on utilitarian grounds.  See e.g. James Fitzjames Stephen, noted 19th-century conservative utilitarian.  You could say, “I’m classifying views based on whether they really maximize total happiness,” but then why include three disparate flavors of “liberalism”?  They can’t all be right.

For utilitarianism to thrive, people need to be comfortable enough to
think of the welfare of others. I believe that 1966 was the period when
whites had the greatest sympathy for the (economic) well-being of
American blacks.

Concern for the welfare of others is part of the utilitarian ethos.  But so is sober cost-benefit analysis and, as a corollary, hostility to Social Desirability Bias.  1966 may have been a period of relatively high sympathy for blacks (though probably little for Indochinese), but it was also an era of rampant wishful thinking.

And America’s middle class was doing very well in the
mid-1960s. As America became more violent in the late 1960s, and more
troubled by unemployment in the 1970s, some of this sympathy dissipated.

Unless I greatly misunderstand Scott, I think he agrees with me that the middle class is doing much better today than in the mid-60s.  So how does this example illustrate the positive effect of prosperity on concern for others?

So I think Bryan’s right that the left/right distinction is not as
meaningful as Scott Alexander assumes, but I also think Scott’s
intuition led him to something important. I don’t know if society is
moving to the left, but I do think it is gradually becoming more
liberal. Is my suggested version an improvement, or not?

I agree that societies that value the utilitarian package – hard-headed pursuit of general happiness – tend to prosper.  But I don’t see much sign that the reverse mechanism works well.  As I originally said, there is plenty of evidence that prosperity makes societies moderate.  That blocks radical changes that harm the general welfare (e.g. mass murder), but also blocks radical changes that help the general welfare (e.g. open borders).

PS. Immigration reform was enacted in 1965.

Yes.  But the liberalization was accidental!