The effect of year of birth on the breast cancer age‐incidence curve in Iceland

Abstract Among different populations, the shape of the age‐incidence curve for breast cancer is strongly related to the overall incidence of breast cancer in the respective population. Data are available from Iceland for the period 1911–1972. These data show that breast cancer has increased very markedly in Iceland during this period, and that as the overall incidence has risen, so the age‐incidence curve has changed in shape, the relation between the shape and the overall incidence being the same as that now observed in other countries. The change in shape is shown to be explicable entirely as a cohort phenomenon, each decade of birth cohort having an age‐incidence curve of similar shape, but with different overall incidence. Data from some other regions of the world indicate that many of the present differences in the shape of the age‐incidence curve may be the reflection of cohort phenomena.