There is a dramatic shift in Palmer and Thornhill's text, starting in chapter 3, and ending at the end of chapter 4. This is the region of the book where the authors discuss the particular evidence they find most relevant to their theses about rape. In this region of the book, they frequently mention issues which they think require more study, and make proposals for tests of their speculations. Somewhat less frequently, but still notably (especially in contrast to the rest of their book), they mention data which seems to raise questions for their hypotheses. The data that they mention in this portion of the book is certainly intriguing and suggestive, though it is quite unclear what exactly to conclude from it.
Since I had wanted not to hate the book, it having been recommended to me by someone I like and have some respect for, I felt very relieved when I started to get into chapter 3, and hoped the rest of the book would continue more in that tone. This looked like science; sure, they were engaging in speculations, and their evidence was far from conclusive, but the phenomena of human behavior are extremely complex. Making speculative claims and proposing further tests seems an entirely appropriate procedure. Unfortunately, there are reasons to worry even about this part of the book.
I will not comment in detail about the quality of the data that Palmer and Thornhill present; I am not by any means an expert in their field. Evolution, Gender, and Rape contains much discussion of their evidence by those with more knowledge of the field. I found the essays in that volume of uneven quality, but some were quite good, and they often cited fascinating evidence and speculations of their own. Some of the essays also raised serious questions about Palmer and Thornhill's methods of analyzing their data, which surely need to be examined; such issues represent the first problem with the more scientific part of Palmer and Thornhill's text.
The second problem is that it ends so quickly. It constitutes roughly a quarter of the book, with the remaining 3/4 devoted to virulent and sometimes dishonest polemic against feminist social psychologists, as well as some very bad philosophy of science.
An obvious speculation is that they did not, in fact, have enough material to make a substantial book, and padded it out with deliberate attempts to provoke controversy (if this was their goal, they certainly succeeded). The next question for my investigation of bias and methodology is whether the flaws in the science section show any sort of pattern, such that Palmer and Thornhill's motivations might have affected that part of the book as well; this question will require far more thorough investigation than I have so far done, so I probably won't be posting any more about this particular book for some time. Which will probably be a relief to many.