Experts aren’t yet sure why colorectal cancer has risen in younger people, but Siegel said it’s an example of the “birth cohort effect”. That people born after the 1950s face heightened risk “tells us that there was some exposure, some risk factor that was introduced in the middle of the 20th century that’s increasing our risk of this disease”, Siegel said, “and it’s increasing the risk more and more with every subsequent generation.”