We found that individual heterogeneity can increase, decrease, or have no effect on population viability, depending on the heterogeneity itself and on how it is maintained over time. This contrasts with some earlier results which have indicated that heterogeneity should always have a positive effect. Given certain assumptions, these can be shown to be special cases of our model. We can analyze several special cases, such as permanent heterogeneity and source-sink dynamics, as well as cases where individual heterogeneity is a function of age or stage. Thus, we provide a general theoretical framework for studying how individual heterogeneity, created by various biological mechanisms, affects the fluctuations of especially small populations.