Intuitively sure, but would be nice to have some data to back it up; my intuition could say otherwise. Also, even if they're different how do we know how the co

Intuitively sure, but would be nice to have some data to back it up; my intuition could say otherwise. Also, even if they're different how do we know how the correlation works? The article claims there is either no correlation or negative correlation. How do we know that?