Frailty, Sickness, and Death: Models of Morbidity and Mortality in Historical Populations

Insurance records from historical sources and recent health surveys suggest that movements in mortality and morbidity are inversely related. Data from nineteenth century England show increases in the prevalence of sickness at all adult ages at the same time that death rates were falling. This paper examines theoretical models which relate movements in mortality and morbidity. The heterogeneous ‘frailty’ model proposed by Vaupel, Manton, and Stallard is adapted to show the effect on morbidity of the increased survival of relatively ‘frail’ individuals. The model is also modified by making frailty endogenously dependent on morbidity. The endogenous frailty or ‘insult accumulation’ model leads to different predictions regarding age patterns of frailty and morbidity and the degree of heterogeneity in the population at birth.