Bowlby researched 44 thieves and interviewed them and their parents to see any signs of maternal deprivation and affectionless psychopathy (Lack of remorse or guilt)
He found 14/44 - affevtionless psychopaths
12/14 - prolonged separation
5/30 - had experienced separation
2 / 44 in control group - experienced long term separation
Concluded prolonged separation or deprivation caused affectionless psychopathy.