People have very high expectations of what psychology can tell us about terrorists. Politicians and security forces want us to identify a “terrorist personality”, so we can identify those who might be attracted to terrorism before they become active. In fact terrorists come in all psychological shapes and sizes. Just like there are musicians with different personalities, and police officers with different personalities, and shopkeepers with different personalities, so there are terrorists with many different personalities, and it is impossible to predict from a personality assessment whether or not someone is likely to become involved in terrorism.
One of the problems of research into terrorism is sampling. Terrorists do not typically volunteer for psychological investigations. Those that do tend to be in custody, and have long ago disengaged from terrorist behaviour.
Horgan argues that the psychology of terrorism is in fact pre-paradigmatic, and that psychologist should focus for the time being on accurate and precise descriptions of terrorist behaviour, rather than attempts at explanation.
This is a hard book, which is definitely aimed at an undergraduate level or above. It will reward those who want to persevere with some advanced reading. That said, there are some highly readable bits and pieces. The snippets of transcriptions of interview with terrorists are fascinating.