Description: This paper, first published in 1982 in the Transactions of Society of Actuaries, Vol. 34, discusses the phenomenon of durational antiselection, a commonly observed steady and seemingly irreversible increase in loss ratios on mature blocks of individual health insurance policies. The author, William F. Bluhm, constructs a model to explain the deterioration of loss ratios over time, and to quantify the cumulative antiselection effect. This paper was awarded the 1980-82 Triennial Prize. The paper includes written discussion and the author’s review of discussion.