There is a drive for more scientists to engage with the lay public. The authors used an augmented version of the theory of planned behavior and identified three factors that predicted scientists' intentions to participate in public engagement activities, over and above their past actions: attitude (whether participation was regarded as positive), perceived behavioral control (beliefs about whether participation was under their control), and descriptive norms (whether scientists believe their colleagues participate). Factors such as career recognition and time constraints did not significantly predict intentions. These findings will contribute to the design of interventions to promote public engagement.