David Canter came up with a circle hypothesis: if you draw a circle with a circumference going through the sites of the two crimes furthest apart from each other, the culprit’s home will likely be near the centre of that circle. Research has shown this to be true of the majority of criminals who strike more than five times. Canter has found that a serial killer can usually be found living within a triangle formed by the sites of his first three murders, as Duffy was.
