Prisons running near capacity have created a dilemma for Andy Burnham. The new prime minister has now confirmed that anyone convicted of manslaughter would not be eligible from the prisoner early-release scheme.

Questions around prison capacity hinge on public safety, of course, but also on what types of criminals “deserve” to be let out early.

There was little pushback when Burnham announced that those convicted of rape, grooming offences or serious child sexual offences would not be eligible for early release. But the revelation that two of PC Andrew Harper’s killers could be released early sparked a passionate public campaign for changes to the scheme. Burnham has now exempted all manslaughter offences, meaning hundreds more people will remain in overcrowded prisons.

The point here is not whether that particular decision was right or wrong, but that there is a tension between what punishment people are thought to deserve, and what evidence suggests may best reduce future offending.

The prison overcrowding crisis shows how politicians struggle to balance evidence on reducing crime with deeply felt and understandable public views about punishment and accountability.

Behind this dilemma are broader questions explored by both philosophy and psychology: what shapes human behaviour, and why do people act as they do? Questions about free will sit at the heart of this debate. In recent research, my colleagues and I explored beliefs on free will, finding that the more strongly someone believes in free will, the harsher the punishment they favour for........

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