I assumed they also controlled for population growth, though actually that's smaller than I would have expected, ~2.9M in 1973, ~3.2M now. I grew up next door i

I assumed they also controlled for population growth, though actually that's smaller than I would have expected, ~2.9M in 1973, ~3.2M now. I grew up next door in Nebraska, and lost my mother and one of her brothers to colon cancer when they were both still quite young (late 40s / early 50s), I think there's certainly some chance of agricultural runoff being implicated (their two siblings who left and never returned are both still with us...)