Assuming stress and urge urinary incontinence (UI) are independent, mixed UI prevalence is 17 times higher than expected. We consider three explanatory models. We summarize evidence from previous studies on whether common Risk Factors, Liability (i.e., one UI subtype increases risk of the other), or Severity model (i.e., mixed UI represents an advanced and more persistent stage of a progressive disorder) explains the unexpectedly high prevalence of mixed UI. We found little support to indicate that the excess prevalence of mixed UI is explained by common risk factors. In contrast, evidence does indicate that onset of one UI subtype increases risk of onset of the other subtype. Finally, although the Severity model is intuitively attractive, there is little epidemiologic evidence to support it. Longitudinal studies are needed to determine which model offers a dominant explanation for the high prevalence of mixed UI.