The likely confounding variable is just age: the oldest segments of the US population are the most caucasian, and these are the populations least like to use sm

The likely confounding variable is just age: the oldest segments of the US population are the most caucasian, and these are the populations least like to use smartphones. So the correlation may be spurious. https://en.wikipedia.org/wiki/Confounding https://en.wikipedia.org/wiki/Confounding