The killing of five people in Plymouth by Jake Davison (pictured), a man whose online activity included regular interaction with the incel sub-culture, has raised questions in the UK around the need for such groups to be targeted under counter-terror and extremist legislation; former Detective Chief Superintendent Gareth Bryon explores the growth in the incel movement and related attacks on women both in the UK and the US, and looks at what can be done to tackle the sub-culture.