I n Roald Dahl’s 1980 masterpiece The Twits, Quentin Blake’s illustrations demonstrate how Mrs Twit’s horrible attitudes eventually ended up deforming her looks. “If a person has ugly thoughts,” wrote Dahl, “it begins to show on the face.”

In her latest book, science writer Helen Pilcher explores this very idea: that negative beliefs “can be physically transformative”. The nocebo effect, as this is known, comes from the Latin for “I will harm”, and strikes when a person’s negative expectations, whether subconscious or conscious, lead to illness.

This Book May Cause Side Effects is a bold attempt to examine the anatomy of this phenomenon. In its simplest form it can be described as follows: “when people are warned to expect symptoms, they become more likely to experience them”.…