Who knows? It could mean some people had a disruption to teeth formation during childhood like a bad fever or disease that makes their teeth break down more. It could mean they had less exposure to mineral strengthening Fluoride in water or toothpaste, and more exposures to acidic drinks or acid reflux conditions. It is hard to prove if genetics play an important role. Many times you see kids with rotten teeth being with parents with no teeth. Could we place it on genetics or that they just don't brush and floss like we recommend and drinking sodas all day long?