People talk as though terrorists are 'other' than us, and while their actions are certainly 'other', experts on terrorists have discovered their backgrounds are often very normal. Terrorists are only human -- too human -- and that can be even more frightening.

Attempts to create a profile of an average terrorist have proved extremely difficult - as difficult as creating a profile of an average 'normal' person. Research into the personality traits of terrorists reveals they have nothing particular in common. Terrorists share the same range of personality traits as might be found in any average office.

The stereotype that terrorists are psychopaths, or are mentally unstable in some way, is a controversial point. The psychologist Jerrold Post has suggested that terrorists are impelled to commit these acts by deep psychological problems. The weight of evidence, however supports the view that terrorists, while they often have very strong religious or ideological beliefs, are generally quite sane.

Heskin (1984) has studied members of the Irish Repulican Army (the IRA), Rasch (1979) a group of West Germany terrorists and Becker (1984), members of the Baader-Meinhoff Gang. All of these found little evidence of psychopathology.

Apart from that it makes sense for terrorist leaders to avoid recruiting psychopaths to their cause: the mentally unstable would prove themselves liabilities to the organisation.

If anything, then, terrorists are notable for their normality, for their ability to blend into the background and remain unnoticed. Those recruited tend to be of average appearance, normal in behaviour in the situation they are in, fairly young - between 20 and 25 - and reasonably well educated, often to university level.
The sociology and psychology of terrorism: who becomes a terrorist and why? [Summary of research, PDF file]
More on London bombings: Psychology of Terrorism, Why We Are Glued To The TV and Guardian Journos Disorientated