In one scenario, the Neolithic revolution comes earlier to some areas than others; those areas then receive their gains in the form of higher population rather than higher wages, for Malthusian reasons. Under some conditions, some of those regions manage slightly positive per capita income growth for extended periods of time (it is on this question that I find the argument both haziest and most parasitic on other theories; toward the end of the book the stress is on whether an economy has had prior selection for “quality” individuals). That can lower their birth rates, which allows for a take-off out of Malthusian constraints. There may be further positive selection for pro-economic growth humans, which compounds and extends growth.
That is not the entire unified theory but it does offer a flavor of which kinds of mechanisms do the work. There isn’t much talk of government policies, coal, or liberal ideology, although every now and then incentives and intellectual property rights appear on a list of factors relevant for growth.
The book has many equations, right in the text, but the main arguments are explained clearly with words.
I would have found it valuable if the author would have asked a concrete question: “Could the Industrial Revolution have come to Song China (Rome, Baghdad, etc.)?” and told us in terms of the parameters of his theory why or why not. I am never sure what stance he is taking on the degree of contingency in observed outcomes.
It is argued that Africa has too much genetic diversity, Native American populations too little. This seems question-begging, and I wonder if the African populations which actually came into contact with each other on a regular basis, pre-imperialism, had so much genetic diversity.
The most valuable part of the book is the extended discussion of how “time since the Neolithic Revolution” matters and how subtle and indirect the indirect mechanisms of connection can be. I consider those discussions to be a major contribution.
It’s certainly an interesting work, but most of the evidence offered is supporting the more general parts of the argument, not the more controversial or novel parts. Galor is very smart, and anyone interested in economic growth should read this book, but I would not describe myself as a convert to either the conclusions or the overall method.
Here is my previous post on the book.