This paper addresses the issue of why, in general, managers choose to voluntarily disclose firm-specific information that is not regulated, e.g. forecasts and environmental information. The literature on the subject has primarily argued that managers choose to voluntary disclose in order to legitimise corporate behaviour and existence or that managers wish to disclose fully but are constrained by considerations of proprietary costs. This paper takes as its departure point the assumptions of self-interest seeking and opportunism as present in positive accounting theory and agency theory. A model is developed and analysed that shows that voluntary disclosure can be a rational response to market estimations of future cash flows under uncertainty that deviates negatively from the self-interest of management. More specifically, management of comparatively strong performers is likely experience a cost due to undervaluation by the equity market that may induce the disclosure of firm-specific information. This, in turn, increases this cost as borne by other similar firms, that then may be induced to disclose the same type of information. The model provides a partial explanation on why managers may wish to disclose firm-specific information under normal circumstances and makes empirical predictions on the diffusion of voluntary disclosure of a particular item.