A problem with correlations: rare traits usually have small correlations to other things, just by virtue of being rare

Correlations provide a very useful, quick way to summarize the relationship between two things (as a single number between -1 and 1). For instance, if we find that self-reported anxiety and self-reported depression have a high correlation (which they, in fact, do), then this suggests a substantial link between the two conditions. But something odd […]