Health psychology deals with individual behavior in a social context. However, within the public health sector, behavior is not restricted to behavior of at-risk persons, but also includes behaviors of peers, parents, health professionals, employers, politicians, and others. Unfortunately, while there is a large amount of empirical data available regarding individual behavior of the at-risk person or patient, there is very little data available about behaviors at other social levels.

Health psychologists try to understand behavior by describing psychosocial determinants for individual behavior. But health psychologists also try to understand and promote behavior change. One basic assumption in health psychology is that to change people's behavior—at least through health promotion interventions—it is necessary to understand the psychosocial determinants of behavior. For example, when a smoker fails to stop smoking because of a lack of motivation, another type of intervention is required than for a smoker that fails because of a lack of social support.