Observed correlations from cross-sectional individual differences research reflect both between-person and within-person correlations

The study of individual differences is often conducted using cross-sectional experiments. The cross-sectional nature of such data means that results obtained from these experiments are a mix of both between- and within-person associations. While concerns about this within-between confounding effect have been surfaced, they are often not addressed when reporting results from individual differences studies. This obfuscation threatens the interpretability and validity of cross-sectional individual differences research. We speculate that a lack of a clear and accessible description of the confounding effect may have contributed to the limited attention to this issue. In this paper, we develop a tractable description of the confounding effect and how it manifests. In doing so, we suggest a need for greater transparency in the reporting of the goals and results of cross-sectional studies and how they contribute to knowledge of individual differences.