The objective of the current study was to systematically review the available literature on the relation between behavioral and cardiac indicators used to measure distress in toddlerhood. After ascertaining the eligibility of 2,424 articles through a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and PROSPERO (International Prospective Register of Systematic Reviews) guided search process, 22 articles ( N = 2,504) that investigated associations between behavioral

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... cardiac indicators of distress in toddlerhood were identified. The narrative synthesis described the overall relation (direction [positive, negative], strength [Cohen's D]) between behavioral and cardiac indicators and was organized by cardiac indicator (i.e., heart rate [HR], heart period, respiratory sinus arrhythmia, pre-ejection period) and type of behavior measured (i.e., coding for expressed emotion behaviors vs. emotion regulation behaviors). Methodological characteristics (i.e., timing of measurement for behavioral and cardiac indicators [concurrent, predictive], length of measurement epochs, inclusion of covariates and moderators) were also described. HR was consistently positively ( D = .05 to .54) related to expressed emotion behaviors. No other cardiac and behavioral indicators were consistently related. Methodological differences related to behavioral and cardiac indicators utilized, timing of measurement, and length of measurement epochs may be responsible for heterogeneity in findings. The findings suggest that researchers might get divergent results depending on whether distress is measured with cardiac or behavioral indicators of distress in toddlerhood. Suggestions for future psychophysiological research with young children are offered.