One-way, mutually constitutive, or two autonomous spheres: what is the relationship between research and policy?

Academics are increasingly exhorted to ensure their research has policy "impact". But is this ambition predicated on an overly simplistic understanding of the policy process? Christina Boswell and Katherine Smith set out four different approaches to theorising the relationship between knowledge and policy and consider what each of these suggests about approaches to incentivising and measuring research impact.