While in the 1970s it was believed that there was a connection between mental illness and terrorism, the current dominant belief, purported by Martha Crenshaw and others, is that terrorists are mentally healthy and rational. This dominant belief has recently been challenged by some publications, such as in particular Kamaldeep Bhui’s work on the connection between depression and isolation and radicalization in UK Muslims, as well as a number of other studies that inquire whether suicide bombers could actually be suicidally depressed, for example.

I argue that both interpretations are correct and that the primary causes for both mental illness and terrorism are the same and include isolation, trauma, exclusion, and deprivation. An individual might become depressed and radicalized due to these factors.

Once radicalized an individual might then join an extremist group which offers inclusion, and may protect against further mental illness. This could account for the fact that we typically find that the normal terrorist is usually not mentally ill. An exception might be the lone wolf terrorist. He or she might remain isolated – by choice or because the opportunities are missing – and might go down the path towards full blown mental illness. In most lone wolf terrorists in history (Ted Kaczynski and Anders Behring Breivik, for example) schizophrenia had been suspected as a diagnosis (even though in both cases it was rejected in court).

Now, the factors that cause mental illness might not necessarily lead to radicalization, which depends on the opportunity structure as well as some other intervening factors. However, the basic processes that are present in mental illness causation might also lay the ground as to why a vulnerable individual might become radicalized.

A case study of schizophrenia is illuminating. Schizophrenia shares many causes with depression. Depression is more likely to be connected to radicalization, as Bhui found.

Schizophrenia is a disease that goes back to ancient times. Even in the bible and other classical texts references to this illness are to be found. In these times, patients were believed to hear the voice of god or to be possessed by demons. The illness is generally characterized by positive and negative symptoms. Positive symptoms include hallucinations and voice hearing and delusions (such as a belief to be under observation or be persecuted and conspired against) and negative symptoms are very similar to depression, apathy and lack of motivation.

The illness started to become a major problem since 1750, with the rise of industrialization. Before, 1 in 1000 was ‘mad’, since then, it rose to 3 in 100 in some countries and the general average is believed to be at something of 1 in 100 worldwide.

Schizophrenia usually occurs in early adulthood (meaning in the 20s) and has a bleak prognosis. Most patients remain without careers or family and have to depend on their parents or institutions to survive. 15% are dead due to suicide after 30 years.

Terrorists are generally believed to be not psychotic (hence schizophrenic). But if we assume that the social and other causes for schizophrenia are comparable to depression, we might be able to make a comparison, as depression has recently been linked to radicalization.

Let’s look at the causes and compare them to terrorism.

Urbanity has been linked to schizophrenia as well as migration, in particular from poor to rich countries. Inequality and poverty also, even though economic downturn could not be established as a cause. These factors tie in neatly, as a migrant to a richer country might end up in the lower strata of society and hence be relatively deprived.

Isolation has been found to be strongly connected to schizophrenia and depression as well. Some sociologists even state that isolation is the main cause for the development of the illness.

Let’s compare that with terrorism. The connection between deprivation has been made by a large number of authors, even though it is highly contested in the literature (in fact, there is a whole debate going on about this issue). I myself have argued in the past that relative deprivation accounts for a large number of terrorists. Isolation has also been established in the literature as a factor, for example with the works of Bhui. Also Borum and Horgan point to this direction, when they state that the need to belong to a group is essential in the decision to join a terrorist group.

Silke finally brings these factors together when he argues that marginalization is a key factor in terrorism causation. Marginalization arguably encompasses both isolation and deprivation. Furthermore, schizophrenia has been connected to trauma, either in early childhood or directly preceding the illness. This has been established in the recent literature as well as in some first person accounts written by patients.

For terrorism, trauma is connected to the literature on psychology of terrorism. Borum reviews the theories on terrorism causation and mentions narcissism and negative identity formation as traditional theories. Both connect to trauma in youth: In narcissism, the inflated ego is traumatized when tested against reality. In negative identity formation, a rejecting father figure plays a role in trauma. Furthermore, it has been found that many terrorists come from disrupted families, with deaths in the families or divorce.

Apart from this, frustration has been linked theoretically to terrorism and political violence more generally. While it is believed in psychology that negative stressors and frustrations of needs fulfillment and goal achievement are responsible for much aggression (Berkowitz, Dollard and Maslow are names to mention here), this has been applied in political science by Ted Robert Gurr to explain why men rebel. He links frustrations caused by relative deprivation to political violence, such as terrorism. Kruglanski specifically mentions trauma as a contributing factor for terrorism.

Finally, let us look at the biological factors in schizophrenia and terrorism.

For schizophrenia, dopamine is a special culprit identified. Dopamine hyperfunction and serotonin hypofunction have been found to explain the positive symptoms, but not the negative symptoms. Cortisol weakness and a lack of oxytocin have also been identified.

For terrorism, there is virtually no study on neurobiology. However, studies have researched the neurological causes of aggression and suicidal behavior, and have found that for both dopamine hyperfunction and serotonin hypofunction are responsible. So, these are the same factors that explain schizophrenia. Cortisol weakness is also identified as a factor in aggression.

So far, we can see that the causal factors seem to be similar. How can we bring them together?

I believe, both mental illness and terrorism are caused by trauma, isolation, exclusion and deprivation. In schizophrenia causation, these are responded to with increasing withdrawal, which is a main feature in this illness. In the causation of terrorism, an individual might chose to resist these stressors, rather than withdraw. If the option of a resistance ideology (blaming someone as responsible for these stressors and as evil) is found, and a group that adopts this ideology, this might be responsible for the road into joining terrorism. If a radicalized person joins a group, this might then protect against further mental illness because inclusion and social support (which are probably present in small clandestine groups) have been shown to protect against mental illness.

I have tried to show how the causes of mental illness and terrorism resemble each other initially, but how they diverge with the decision to take up a radical ideology and join a terrorist group.

If my hypothesis is true, that would mean we could potentially find a way to screen for vulnerable individuals. It might also mean that it is possible to determine regions and countries more at risk given their prevalence of mental illness. A country with higher rates of depression, for example, might be likely to produce more radicalized individuals. It might even be possible to develop an early warning system from here on. If we know that individuals need to become depressed first before they join terrorist groups, depression rates might increase before violence breaks out. Therefore, looking at depression rates might help us determine which countries are at greatest risk of violence.

Data on depression confirm this interpretation. Countries with current conflicts, in particular in the MENA region, are showing extreme levels of depression. This would confirm my interpretation at least that depression and conflict co-occur. Also, in the cases of Middle Eastern countries, depression rates rose before the onset of conflict and violence and remained high. To compare, in Bosnia, depression rates were high before the crisis of 1993, but fell significantly afterwards. Germany and Japan, as controls, have relatively stable depression rates.

We could also look at suicide rates for establishing a link. Suicide seems to occur before war more than in war. In both Germany and the US, suicide rates were extremely high before the First World War, in the US also before the Second World War, but fell significantly during the wars. Suicide is to 90% caused by depression. However, the WHO, who counted suicide rates, does not necessarily confirm my hypothesis, nor does their data on suicide match with the afore mentioned depression data. In particular, Middle Eastern countries seem to be strangely protected against suicide, even though they seem to have high depression rates. This could possibly be explained with cultural factors in the region. Collectivism means that a mentally ill individual might remain more integrated in society, which is known to protect against suicide.

To conclude, I would like to mention that this opens up room for thinking about medical approaches to conflict. If mental health is at the root of radicalization and terrorism, then medical approaches could be fruitful for countering these issues. They would certainly be more ethical than military approaches.

Finally, I conclude with a call to establish the discipline of International Political Psychology. Such a new approach is needed to research topics such as this in more detail.