They employ characters and a narrative arc, and draw moral lessons. They manage to persuade by appealing to common experience and explaining familiar events, by creating a shock of recognition in their audiences, a sudden realisation that ‘this must be true’. Such theories – by virtue of the very fact that they seek to encompass the human – must always go beyond their evidence.

What makes claims about human nature become truisms? How do they gain credibility? They might rely on experiments, case studies or observation, but evidence alone is never enough to persuade. Despite the controversy they provoked, such theories often attained the status of conventional wisdom.

The social psychologist Albert Bandura countered that aggression was not inborn but resulted from imitation and suggestion. The neuroscientist Paul MacLean maintained that humans’ violent tendencies could be traced to their primitive ‘reptilian brain’. The founder of psychoanalysis Sigmund Freud argued that ‘man is a wolf to man’, driven to hatred, destruction and death. Horrified by the atrocities of the 20th century, an array of scientists sought to explain why human beings turned to violence. 0:01 Brought to you by Curio, a Psyche partner The killer-instinct idea achieved such cultural power because it came embedded in gripping stories about human nature

Primal instinct meaning