Dunblane, Virginia Tech, Sandy Hook – all mass murders committed by paranoid individuals. These events feature prominently in psychologist David LaPorte’s Paranoid, in which he attempts to give an account of what paranoia is, and why it happens.

Paranoia – defined by LaPorte as “excessive, undue, or unreasonable suspiciousness” – is surprisingly common. Some 15 to 30 per cent of people regularly experience paranoid thoughts; 8 to 10 per cent suffer from delusions of persecution. Clearly (and fortunately), few of those experiencing paranoia go on to commit atrocities.

What causes paranoia? LaPorte offers several explanations, focusing on his hypothesised “suspiciousness system”. Drawing on evolutionary psychology, he notes that humans evolved in small, close-knit social groups. Deviations from this – eg, urban living – increase baseline levels of “suspiciousness”, pushing some into the pathological (paranoid) range. Threatening environments, such as the post-9/11 US, don’t help.

But I was not convinced that LaPorte’s “suspiciousness system” provides much explanatory power. Living in threatening environments will increase suspiciousness – but whether this is maladaptive, as paranoia is, depends upon the context. Being suspicious in the former East Germany, where one-third of people were secret informants, sounds reasonable. But what about those who show such mistrust in our modern, liberal society? To take another example, sensory impairments, such as hearing loss in the elderly, correlate with paranoia. But why do some accept that their hearing is failing, while others think that they are being whispered about?

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The really interesting question, which he doesn’t really address, concerns the cognitive biases influencing our individual interpretations of ambiguous situations. LaPorte attempts a neurobiological account of suspiciousness – suggesting a key role for dopamine, based on findings that antipsychotic medications reduce paranoia. But as medical students know, you can’t necessarily link a drug’s mechanism of action to a disorder’s cause; headache isn’t caused by aspirin deficiency.

Admittedly, paranoid individuals are hard to study. Their chronic mistrust makes them unlikely volunteers for research. But if LaPorte’s “suspiciousness system” is right, then there may be value in studying those lower on the suspiciousness spectrum – but this is not a point that he pursues. Instead, he makes a (valid) plea for cross-disciplinary research; paranoia features in many psychiatric and neurological disorders (eg, schizophrenia and Alzheimer’s disease) and yet it is usually subsumed within the larger disorder diagnosis. Breaking down disorder boundaries to focus on symptoms (and, arguably, the underlying psychological and neurobiological mechanisms) could be extremely productive.

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There is much to like about this engagingly written book. LaPorte’s extensive knowledge of paranoid cases informs fascinating vignettes. The overall effect is of having an engrossing conversation with a psychiatrist friend over dinner. But it could have been much more. LaPorte’s aim is to “describe exactly what paranoia is”, but what I want is an explanation, and the book raises more questions than it answers. But that’s true of all official explanations for good conspiracy theories – so maybe that’s exactly what LaPorte wants us to think.

Amy L. Milton is lecturer in the department of psychology, University of Cambridge.

Paranoid: Exploring Suspicion from the Dubious to the Delusional
By David J. LaPorte
Prometheus, 290pp, £14.99
ISBN 9781633880689
Published 18 September 2015