Diamond is very good at making complex theories accessible and interesting to lay people. Sadly, in the process, he destroys his own credibility as a scientist, by never *quite* joining up his chains of reasoning, and by making a few sweeping (and absurd) statements that lack any kind of rigorous scientific approach.
Most glaringly, for example, he starts off his Prologue discussing alternative theories about why some societies developed faster and further than others. On Page 19, he declares how "loathsome" a "racist" explanation based on average intelligence would be. While such an explanation is almost certainly *incorrect*, there is no place in science for assessing *facts* on the basis of whether they are liked or not. One can't help wondering if Diamond would have shrunk from the facts, had the evidence in fact pointed to an explanation based on relative intelligence.
But then on the following few pages (and almost amusingly), Diamond then goes to great pains to show that in his opinion, New Guineans are on average more intelligent than Westerners! So it seems that in his mind, the loathsomeness of such racial generalisations cuts only one way. And of course, one wonders how he reached such a conclusion - since the measurement of intellience is very tricky even for experts in that field! Certainly he quotes no research on the subject, neither his own, nor that of others.
The rest of the book also lacks a rigorous scientific approach - Diamond makes dozens of small errors, ranging from the way he expresses statistics, to non-sequiturs in a chain of reasoning. In my opinion, they do not fatally flaw the gist of his thesis, but they do convert it from a coherent scientific theory into what is merely a good story.
It's not impossible to write a book that is entertaining and readable *and* logically complete, and scientifically sound - as evidenced by authors such as Steven Pinker, Isaac Asimov, and Richard Dawkins, to name but a few.
I gave it a rating of only 2 because despite being an intriguing and important topic it leaves a scientifically critical reader wondering how valid the conclusions are, even though one *senses* that Diamond is spot-on in his explanations. And that's a disappointing waste.