Lying is ubiquitous and has acquired many names. In ‘natural experiments’, both pathological lying and truthfulness implicate prefrontal cortices. Recently, the advent of functional neuroimaging has allowed investigators to study deception in the non-pathological state. Prefrontal cortices are again implicated, although the regions identified vary across experiments. Forensic application of such technology (to the detection of deceit) requires the solution of tractable technical problems. Whether we ‘should’ detect deception remains an ethical problem: one for societies to resolve. However, such a procedure would only appear to be ethical when subjects volunteer to participate, as might occur during the investigation of alleged miscarriages of justice. We demonstrate how this might be approached.