The practice of safety can be treated as the operation of a system with which it is intended to control the level of some parameter ‘risk’. The following is an attempt at a description of the behaviour of a population of individuals engaged in a risky occupation in terms of such a system. In the first section the casualty figures for commercial airline operations are analysed to show that there is a prima facie case for a belief that in this case risk level is regulated by those immediately engaged in the industry. The second section contains a description of a proposed ‘input/output’ model of an individual regulating his own risk, and preliminary results are given for a simulated population of aircraft operators having these characteristics. Lastly, the credibility of the model in terms of known human behaviour is examined, some tentative conclusions are drawn and an indication is given of its proposed use and development.