According to a study described in this article they are...
http://www.theguardian.com/commentisfree/2015/dec/03/scientists-easy-prey-jihadis-terrorists-engineering-mindsetThe terrorists’ black and white worldview appeals to an ‘engineering mindset’, a study suggests. A broader education would give vulnerable students the tools to question authority
What kind of person becomes a jihadi terrorist? Specifically, what kind of educated person? The overwhelming majority of graduates recruited into Islamist terrorism studied engineering, science and medicine. Almost none are social science or arts graduates, according to research. The insight could have important implications.
Almost half (48.5%) of jihadis recruited in the Middle East and north Africa had a higher education of some sort, according to a 2007 analysis by Diego Gambetta that is cited in Immunising the Mind, a new paper published by the British Council; of these 44% had degrees in engineering. Among western-recruited jihadis that figure rose to 59%.
A study of terrorists in Tunisia – where an electrical engineer went on a murderous rampage in June – showed similar proportions. And a study of 18 British Muslims implicated in terrorist attacks found that eight had studied engineering or IT, and four more science, pharmacy and maths; only one had studied humanities.