It's not only their diet and lifestyle, it's their genetic background, where they live, their age, and general health, as well. Lab scientists would much rather study these things in strains of in-bred mice, where all the mice are genetically identical, they all eat the same food, and live in the same place. That way we don't have to rely so much on statistics to explain away sampling errors.
We don't have in-bred humans… outside of West Virginia
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