Before I get to the "unclear" part, let me first say that as one of the few books that deal with the psychology of eating, drinking, and food in general, the range of topics covered in this book is pretty good. The author attempts to incorporate areas as diverse as the psychology, physiology, and cultural anthropology of eating, drinking, and food. On top of that, she also adds quite a bit of personal anecdotes to her writing, which can be quite endearing (even though as an adventurous and non-fussy eater myself, I find her pickiness and her narrow-mindedness when it comes to sampling food that is not American a little annoying).
I gave the book this low rating not because of the aforementioned minor defect. I think the book's biggest problem is that, whenever Logue tries to explain in detail any scientific study, she becomes very unclear and very confusing. There were many times when I had to read and re-read a particular description of a study, but I still could not understand what the study was about. This is really quite a shame because I think Logue did select some seminal studies to mention. It is just that she has fallen short of communicating them with her readers well enough.
If this book is indeed intended for an "intelligent lay audience" as pointed out by one of its reviewers, then it needs to do an even better job in explaining scientific findings to its audience. I am not a "lay person" but even I found certain parts of the book very confusing.