One of the main problems with custodial sentencing is the rate of re-offending, which is known as recidivism and looks at how likely it is that once released from prison, someone will commit another crime and end up back in there. According to the Law of Effect, any behaviour that is followed by pleasant consequences is likely to be repeated and therefore the best way to change behaviour is to try to associate the desired behaviour with something pleasant, which is the basic idea behind behaviour modification. An example of behaviour modification is the token economy system, which works on the basis of operant conditioning in that desirable behaviour can be exchanged for some form of token, which can in turn be exchanged for some form of privilege. The use of behaviour modification has been shown to be effective in the treatment of schizophrenia and in changing behaviour in schools.