Paris: 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.