Paris (AFP) - Experts say the link between mental illness and so-called "lone wolf" terrorists is driven by the fact that unstable individuals are often influenced by events in the news, a fact that is exploited by global jihadist groups.
Tuesday's knife attack by a 27-year-old German shouting "Allahu Akbar" left one dead and three injured in Munich.
But police quickly dismissed any jihadist motive, saying there were "strong reasons" to believe he acted "in a state of insanity".
Numerous similar cases have been reported around the globe.
Man Haron Monis, who died along with two of his hostages at a Sydney coffee shop in December 2014, had a long history of mental illness.
So did Michael Zehaf-Bibeau, who killed a Canadian soldier near Ottawa's parliament two months earlier.
Experts say the connection is not unexpected, since the jihadist ideology offers a compelling narrative for dealing with feelings of marginalisation and paranoid fantasies of persecution that can exist among people with severe mental illnesses.
"Each time society evolves, delusional people evolve. Delusional behaviour is always connected to the times," said psychiatrist Daniel Zagury, who has acted as an expert witness at the trials of several alleged jihadists.
"There have always been mystical delusions. They are often the most dangerous. When God is on your side, things become much simpler," he added.
"Today, it's 'Allahu Akbar' that gives a sense of the mystical, of the messianic, to their actions. That's why we have these people driving their cars into crowds or stabbing strangers: the news has fuelled their schizophrenia, their delusional outbursts."
Zagury warned against labelling all jihadists as psychologically unstable -- saying they account for only around 10 percent of cases.
The majority are either "small-time delinquents... who started off as drug addicts, dealers, and try to clean up their lives by turning to radical Islam."
Or they are "the most dangerous kind" -- the clean-living, well-educated youngster who becomes a true believer in violent extremism.
- 'A vengeful Allah' -
But often the line between true believer and mental instability is blurred, and there have been few comprehensive psychological studies on jihadists to unpack the complex mental processes involved.
"We often tend to say that these people are unstable, but we need a proper study. Every case is different," said clinical psychologist Amelie Boukhobza.
"We can easily have someone who is close to the radical Islam movement and also has psychological problems," she told AFP.
Boukhobza said the constant spotlight on the Islamic State group (IS) in the media has undoubtedly influenced some people with severe mental illness.
"For most delusion psychotics, God is present -- it can be either a kind or evil God.
"With the phenomenon of Daesh (another name for IS) growing so much, God can be replaced by Allah -- an even more present, vengeful Allah," she added.
IS has understood the advantage of constantly calling for random acts of violence, added Patrick Amoyel, a psychoanalyst who works on radicalisation issues at Sophia-Antipolis University in Nice, France.
"They know that the more they dominate the media, the more they will get a reaction, either from people who are vulnerable to radicalisation, or from psychopaths.
IS "represents the anti-society, the anti-West, which can channel a sort of social radicalism that doesn't necessarily have anything to do with politics or religion," added Amoyel.
"There are those who know what they are doing... who are real terrorists who are acting rationally," he said.
"But there are also those who have psychopathologies... and Daesh's directives can push them to act."