There has been considerable interest in how we ascertain whether an environment is controllable and the neural mechanisms mediating this process. An especially acute version of this problem occurs when multiple people work together. Using a new task, fMRI and computational modelling, we demonstrate that in such ambiguous social contexts, people engage in specific behaviours that we refer to as active disambiguation. This process helps individuals establish what they themselves, as opposed to others, control and what consequence they themselves cause or that another person causes. People identify when active disambiguation is needed and engage in it at that time. A pattern of activity in the supramarginal gyrus that emerges during and after active disambiguation is linked to tracking uncertainty and establishing controllability. We show that activity in this brain region signals a second learning mechanism, by which individuals attribute outcomes to themselves versus others, proportional to their perceived control.