"It"s something that"s repetitive and soothing but even in the latest Diagnostic and Statistical Manual of Mental Disorders [DSM 5 by the American Psychiatric Association], the "psychiatric Bible", these sorts of behaviours are classified as purposeless," says Steven Kapp, a developmental psychologist at the University of Portsmouth who was diagnosed with autism aged 13. "So, I think a lot of researchers have struggled to understand why people engage in these behaviours."