Whoa, big book. 680 pages in all, but fortunately "only" 489 of them are the main text. Lots of notes.
To get it out of the way: the WEIRD in Joseph Henrich's title stands for "Western, Industrialized, Educated, Rich, Democratic". In other words, us. (Well, me, anyway. Probably you.)
Henrich believes that our brains work significantly different from the rest of the world, and this difference has been accumulating over the past couple thousand years. (He is properly scornful of psychology researchers who try to pass off their experimental results as broadly applicable, but only really apply to their research guinea pigs, typically American college undergrads.) He supports this thesis with a raft of data drawn from his own work (he's chair of the Human Evolutionary Biology Department at Harvard) and disparate social sciences: history, economics, anthropology, sociology, etc.
What happened to make us this way? Christianity, mostly, specifically the Catholic Church. Their efforts over centuries to impose their "Marriage and Family Program" (Henrich acronymizes this to "MFP") turned the flock away from mankind's usual tribalism. Other things happened too, of course, but Henrich finds this to be the biggie. It's plausible: the MFP reverberates thoughout Western society, causing culture to evolve in unexpected and unintended ways. But mostly it changed our brains in a WEIRD way…
Fortunately, there's a diagram:
[That's, um, borrowed from this site; if you're interested in a much more detailed summary of the book, click over.]
While reading, I kept wondering "What would Deirdre McCloskey think about all this?" One of the primary themes here is the development of modern capitalism, after all, right in Deirdre's wheelhouse. There is but a single footnote to one of Deirdre's books here. Surprising!
Henrich's style is mostly dry, but not without occasional flashes of humor. Example: on page 273 he's discussing how the MFP's encouragement of monogamy affects testosterone levels in men. "Here, you're seeing how the Church, through the institution of monogamous marriage, reached down and grabbed men by the testicles." Ha! (Although I think I crossed my legs when reading that.) I suspect this and other zingers are a crossover from Henrich's undergraduate lectures at Harvard; even those bright kids can get their attention perked by a good joke.