This book is basically Jared Diamond meets The Bell Curve. Hart's thesis is that we can't really understand human history unless we allow for human biodiversity. The result is an intellectual bitch-slap to the mainstream PC consensus (I understand why many people find this argument troubling, but troubling does not equate with untrue, let's accept the obvious truth and deal with it).
The first part of the book basically summarizes differences in average IQ across racial groups. Hart correctly points out that very few people have truly accepted evolution, most are still creationists of some sort, including those "who say (and think) that they accept the theory of evolution, but who in fact shrink from accepting the implications of the theory. Among the unwelcome implications are: 1) Human beings are animals . . . 2) Evolution is a completely amoral process. 3) A person's physical capabilities and limitations are strongly influenced by his genes. 4) A person's mental attributes (i.e., his individual abilities and proclivities) are also influenced by his genes – not rigidly determined, but strongly influenced . . . 5) The observed behavioral differences between the sexes are strongly influenced by our genes . . . 6) Whenever two populations within a species are reproductively isolated, they will diverge from each other genetically . . . 7) The process of evolution did not stop with the emergence of Homo sapiens . . . 8) There is no reason to suppose that the visible differences we see between the regional variations of human beings are the only differences that exist between them. On the contrary, it would be very surprising if that were the case."
Hart then gives us tons and tons of data and observation to support the point that average IQ varies across racial groups (he traces these groups to different migrations out of Africa). Hart spends some time discussing the incredible success of the speakers of proto-Indo-European.
Then he gets into scientific and cultural advances. The dominance of northern Europeans is hard to argue with. Here some of Jared Diamond's arguments are certainly valid (Diamond believes that intelligence explains nothing (in fact, to Diamond, we're all of equal intelligence, except for people living in the New Guinea highlands, who are smarter than all of us) and that geography and the presence of plants and animals that can be domesticated explains all of human history). Hart points to several problems with this thesis, all of which can be better explained by introduced human intelligence as another variable in the equation. For example, according to Diamond's thesis sub-Saharan Africa should have been farther along (in terms of development) than South America immediately prior to Columbus's discovery. Instead the opposite was the case (by a lot). IQ differences in the populations could explain this discrepancy. (The sub-Saharan Africans hadn't even settled Madagascar which was 250 miles off their coast; instead, the island was settled by people coming from Indonesia – over 3000 miles away).
Hart does not put all his eggs in the IQ basket. He is only suggesting that leaving IQ out of the mix results in obviously incorrect answers.
I think this gives you a taste of his arguments, but the book is worth reading for the incredible scope and interesting analysis. You get a short summary of all of human history, which is quite well-written. The discussion on why Europe advanced much faster than China (where average IQ was probably higher) is also very interesting and relies in part on arguments like Diamond's.
The only omission, in my opinion, was some mention of standard deviations. Let's say one human group had an average IQ of 85 and another had an average IQ of 100. Do we expect standard deviations of IQ to be proportional across all these populations? I suppose so, if it's truly a bell curve, but do we know that this is true? Either way, what type of IQ is necessary for an Aristotle or a Newton? Is genius all that matters, or must genius sprout in a place that has other intelligent people (above some IQ threshold)? Some analysis of this would help fill in some gaps.