Introduction
Europe has witnessed a number of waves of terrorism over the last decades. Starting from the 1970’s, we have seen the Rote Armee Fraktion (Red Army Fraction) in Germany, the Brigade Rosse (Red Brigade) in Italy, the ETA (Basque National Liberation Movement) in Spain and France and the Irish Republican Army in Ireland/UK. This is just a list of the most prominent groups that were willing to use violence against non-combatants to achieve political and/or societal changes. More recently, we have been exposed to violence from extreme Islamic groups (such as Al Qaida and ISIS) in various European countries as well as extreme right wing groups (such as Pegida in Germany). What can Social Psychology contribute to the understanding of terrorism, in terms of roots and consequences?

A Role for Social Psychology?
First, people are not “naturally born killers”. As elegantly demonstrated with historical empirical evidence, Gossmann (1996) shows that military people, in close combat, often have chosen NOT to shoot the enemy. People need to be convinced that their harmful actions towards another human being serve a greater purpose, for example, the liberty of a nation or the freedom of a people.
Secondly, it is clear that terrorism is very much a group phenomenon. More than 95% of terrorist attacks are done by groups. Being in a group, such a military group, makes it easier for people to display violence than when being alone.
Thus, terrorism is often driven by a combination of cause and comrades (McCauley, 2002). The cause is about an ideology and the comrades are about group membership. Both aspects are core business in Social Psychology (e.g., Doosje, B., Moghaddam, Kruglanski, de Wolf, Mann, & Feddes, 2016): it is about the need to belong to a distinctive group, leadership, group pressure, in-out-group differences (e.g., dehumanization of the out-group), and sacrifice for the greater good of the group and the cause (quest for significance – see , and often times it can be seen that certain groups in conflict “need” each other to justify the violence (see Reicher & Haslam, 2016).

Limitations of Social Psychological Research
While it is possible to link certain theories, notions and findings from Social Psychology to the issue of terrorism, it is not that easy to study them in a clear empirical manner as we might want to. First, terrorism is rare. Thus, there are not many terrorist in the world. Second, it is difficult, if not impossible, to get terrorists or potential terrorists in a psychological lab study. Third, field experiments are difficult as well for the same reasons: terrorist are not that willing to be a participant. In response to these difficulties, psychologists have focused their attention to a broader segment: Radical people. Again, only a limited number of radical people continue to display violence (see the influential staircase to terrorism metaphor by Moghaddam, 2005).

Other Disciplines
Thus, what you see more often is that people with other research traditions, such as anthropologists or political scientists, study them with more qualitative methods, such as interviews or observation. But before you can do this, it takes time to build a relationship of trust with the participant.
Another manner to study terrorist is by trying to sketch the pathway from an innocent childhood to a violent adult. This includes doing archival studies and interviewing people who were in contact with the terrorist and sometimes the (sometimes former) terrorists themselves. Obviously, interviewing terrorists themselves raises the question of social desirability as well as tactics to use scientists (and the media) to distribute radical ideas and gain support.

Scattered Knowledge
Perhaps even more than with other subjects, terrorism is being studied from so many angles and thus by different disciplines. It has relations with psychology, sociology, anthropology, political science, security studies etc. Ultimately, this leads to a field filled with scattered knowledge. As scientist (Social Psychologists or otherwise) it is our task to gain as much knowledge as possible from other fields to come to a better understanding of terrorism, its roots and its consequences. We have to accept the limitations of doing research in this area and accept the roles of other disciplines.

• Doosje, B., Moghaddam, F. M., Kruglanski, A. W., de Wolf, A., Mann, L., & Feddes, A. R. (2016). Terrorism, radicalization and de-radicalization. Current Opinion in Psychology, 11, 79-84.
• Grossmann, D. (1995). On killing: The psychological cost of learning to kill in war and society. Boston: Little, Brown and Company.
• Kruglanski, A. W., Gelfand, M. J., Bélanger, J. J., Sheveland, A., Hetiarachchi, M., & Gunaratna, R. (2014). The psychology of radicalization and deradicalization: How significance quest impacts violent extremism. Political Psychology, 35, 69-93.
• McCauley, C. (2002). Psychological issues in understanding terrorism and the response to terrorism. In Stout, C. E., & Schwab, K. (Eds), The psychology of Terrorism, Vol 3. (pp 33–65). Westport, CT: Praeger.
• Moghaddam, F. M. (2005). The staircase to terrorism: A psychological exploration. American Psychologist, 60, 161–169
• Reicher, S. D., & Haslam, S. A. (2016). Fueling extremes. Scientific American Mind, 27, 34-39.

E.J.Doosje@uva.nl