Abstract for: System Dynamics and GMB in Practice – the Case of Heavy Cannabis in The Netherlands

At the Ministry of Security and Justice in The Netherlands, System Dynamics and group model building are used to estimate possible effects of new policy measures (ex-ante impact analysis). One example is a measure (not yet confirmed by the Dutch parliament) to make a distinction in criminal law between ‘soft’ and ‘heavy’ cannabis, depending on the concentration of its active ingredient THC. When the law becomes operative, higher sanctions will be imposed on the production, trade and possession of ‘heavy’ cannabis. SD and GMB were used to gain insight in the effects of this measure on the behaviour of users, coffee shops and illegal markets, and on the extra workload for actors in the criminal justice chain.