Here are responses to the Reading Club AMA questions, point-by-point:

December:

What about the possibility that the distribution of traits may vary
between populations due to genetic differences between populations? If
it is those traits are driving long-run growth, and those traits cannot
be easily copied due to their partial genetic basis, how does that
affect the analysis you’ve been making in this series and your views on
open borders?

The papers covered in the reading club treat ancestry as fixed, but remain agnostic on the mechanism.  It could be genetics; it could just be persistent culture.  The mechanism doesn’t really affect their results.  However, my claim that civilized migration outperforms historic migration by swiftly acculturating newcomers does depend on the mechanism.  The greater the role of genes, the less credible my revisionist story becomes.

In general, I am very open to genetic explanations of human behavior; I have a whole book on the topic.  But as I explain in that book, virtually all behavioral genetic evidence measures the effect of genes within the First World.  The fact that environmental differences within the First World have little long-run effect on human development is a flimsy reason to doubt that growing up in the Third World is tremendously damaging.  And as usual with migration, this is not just a humanitarian concern; physical and intellectual stunting also hurts the global poor’s potential customers.

Denver:

While I tend to agree with your assessments, I do have one
reservation. Most of these measures are simply looking at the ancestry
of the populace, and correlating it with current levels of productivity.
But what about institutional impacts? That is, what impact do you think
ancestry plays in forming social norms and government policy? Which could then have these observable impacts in productivity.

The papers I discussed present “reduced form results”; they sum all direct and indirect effects of ancestry, institutional impacts included.

For example, if ancestry measures were as predictive as
anti-immigration advocates often imply, then we would expect to see
homogeneous nations (in terms of ancestry), like Europe, outperform
their heterogeneous counterparts: the US. On the whole, I tend to think
the opposite is true: heterogeneity outperforms homogeneity.

Indeed, this is just what Putterman and Weil find.

…So I have one final question: given
the two options, open borders or open borders given that immigrants from
backwards countries (measured by “SAT”, IQ, or another relevant
measure), and their descendants, are barred from voting, which would you
choose?

I would let them vote.  Voting restrictions make sense as long as relatively sensible people care about the well-being of less-sensible people, leading voters to pick policies that are better for everyone.  Contrary to popular belief, this is roughly true for U.S. citizens.  Unfortunately, unselfish voting stops at the nation’s border.  A century of U.S. policy shows native-born Americans can’t be trusted to treat foreigners with minimal decency, much less take their interests to heart.  Giving foreigners the vote helps them collectively defend themselves from natives’ callousness.  While this raises the risk foreigners will treat natives unjustly, the opposite danger is far more serious.

Joshua Woods:

1. What seems to you to be the most promising avenue for future research in this area?

Re-doing all the results with population weighting, so we stop treating China, India, and the United States as empirically relevant as Bhutan, Malta, and Belize.  Nathaniel Bechhofer is pursuing this route already.  You’ll know more when I do.

2. The addition of latitude controls drastically affected the
coefficients on the various ancestry variables. Do you think this
powerful effect of latitude holds today at our much higher technology
level?

My best guess is that there’s a huge indirect effect.  Latitude mattered a lot directly in the past.  But no matter why a country is poor today, current poverty holds back prosperity in a hundred different ways.

Ben Wilson:

Has any objective research or even informed speculation been done on why
latitude effects are so strong? Agriculture concerns would have been
important in past centuries, but probably not so much in the past 50
years. Plucky New Englanders might suggest that experiencing the seasons
sets our resolve to be more productive (or something).

Easterly and Levine 2003 try to do this.  Their bottom line: “We find evidence that tropics, germs, and crops affect development
through institutions. We find no evidence that tropics, germs, and crops
affect country incomes directly other than through institutions, nor do
we find any effect of policies on development once we control for
institutions.”  I find their distinction between institutions and policies pretty artificial, but their paper is still the obvious place to start.