Neuroscientific endeavours to uncover the causes of severe mental impairments may be viewed as supporting arguments for capacity-based mental health laws that enable compulsory detention and treatment. This article explores the tensions between clinical, human rights and legal concepts of “capacity”. It is argued that capacity-based mental health laws, rather than providing a progressive approach to law reform, may simply reinforce presumptions that those with mental impairments completely lack decision-making capacity and thereby should not be afforded legal capacity. A better approach may be to shift the current focus on notions of capacity to socio-economic obligations under the Convention on the Rights of Persons with Disabilities.