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The aperiodic "slope" of the EEG power spectrum (i.e., aperiodic exponent, commonly represented as a slope in log-log space) is hypothesized to index the cortical excitatory-inhibitory balance. Slope has been associated with various neurodevelopmental outcomes in older children and adults, as well as with family history of ADHD in infants. Here, we investigate associations among EEG aperiodic slope, temperament, and maternal internalizing (anxiety and depression) symptoms in a large cohort of typically developing infants. A steeper slope was associated with higher scores on the temperament domains of orienting/regulation and surgency but was not associated with negative affectivity. Maternal symptoms did not appear to be directly associated with the slope, but the slope moderated the association between maternal symptoms and temperament. Specifically, a steeper slope was associated with a stronger negative association between maternal internalizing symptoms and infant orienting/regulation. These results demonstrate associations between slope and behavior as early as infancy, which may reflect early differences in the development of global inhibitory networks. Longitudinal research in early childhood is necessary to better understand the nature of these relations during development and their potential impact on later socioemotional outcomes., (© 2025 Society for Psychophysiological Research.)