Obviously there's a genetic component and certain people get an unlucky dice roll no matter what they do...
But I still think it'd be worth them looking into potential links to overuse of antibiotics and the massive uptick in SSRI usage in young people that was happening in the 90's. (both of which have an impact on the gut biome)
The stat I referenced earlier, a person born in 1990 (statistically) has double the likelihood of getting colorectal cancers compared to their parents and grandparents, that's not insignificant, and can't be explained exclusively by changing diets or bad luck.
SSRIs has been studied (Pubmed search search term: SSRI colorectal cancer), no one has reported an increase in risk. All but one report a protective effect (the one reports no effect)
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There were some results that focused more on potential mechanism instead of epidemiology. Please check that I didn't miss anything relevant.
This is of course provisional, future findings can change the picture. As it stands now it seems that SSRIs don't increase the risk of colorectal cancer.
It's not as if human genetic lines magically changed that much in the transition phase from new-wave to grunge to make that many people more susceptible to bad luck or a few extra Big Macs.
There had to be some sort of outside catalyst that was introduced driving some of it.