Between- and within-person contributions of simple reaction time to executive function skills in early childhood.

This study tested whether the bivariate association between simple reaction time (SRT) and executive function (EF) performance that has been observed in early childhood represented a between- and/or within-person association. Up to three repeated assessments (i.e., fall, winter, and spring assessments from September to May) were available for 282 preschool-aged children (M = 4.2 years; 54% female) who participated in the Kids Activity and Learning Study. A series of three-level hierarchical linear models (repeated measures nested in child; child nested in classroom) was used to disaggregate the observed variation in EF and SRT into between-classroom, between-person, and within-person components. EF composite scores were regressed on two indicators of SRT, which reflected between- and within-child sources of variation, along with demographic covariates (child age, gender, and parental education). Both between-person (b = -21.2, p < 0.001) and within-person (b = -13.2, p < 0.001) sources of SRT variation were uniquely related to EF performance. These results are discussed with respect to interest in using SRT as a proxy for foundational cognitive processes that contribute to EF task performance in early childhood, including the appropriateness of using SRT to refine EF task scores.