Correlation and explaining variance: To square or not to square?

Despite previous articles dating back 80 years, the questions of whether and when to square correlations continue to puzzle and confuse researchers. In this editorial, I point out that correlations can serve two independent purposes: they can be measures of effect size in themselves and their function as regression coefficients can be used to estimate proportion of variance in one measure for which another measure accounts. Using examples relevant to intelligence researchers, I show that the answer to the question of whether or not to square a correlation is ‘it depends.’ It depends on the purpose and it depends on the underlying theoretical model of the causal association between the variables.