["Developmental Science, Volume 29, Issue 4, July 2026. ", "\nABSTRACT\n\nTheories of emotion development propose that negative affect decreases over time as attention control and emotion regulation improve. Yet, research on infancy suggests that the first year of life can be characterised by a different developmental pattern: increased negative affect as infants age. Implementing an emotion‐eliciting task, we measured infants’ observed emotion reactivity and regulation, and physiological arousal (heart rate) during the challenge (toy retraction) and recovery (toy play) conditions at age 6 months (n = 82, 43 boys) and age 12 months (n = 68, 36 boys) continuously on a second‐by‐second basis. We first measured mean intensity of reactivity, regulation, and arousal by calculating within‐person averages. Then, we measured three temporal dynamics: estimates of variability by calculating variance, lability by calculating root mean square of successive differences, and persistence of changes by calculating autocorrelation. Intensity, variability, and lability in negative reactivity increased from 6 to 12 months with no change in persistence, indicating a larger and quicker negative reaction to toy retraction with age. Regulatory strategy use increased from 6 to 12 months during challenge, whereas during recovery, it reduced and became more stable. Heart rate decreased from challenge to recovery at 12 months but not at 6 months. We then examined how the temporal associations between reactivity, regulation and arousal change over time. Cross‐correlation analyses revealed stronger temporal associations between reactivity, regulation, and heart rate at 12 months, suggesting increased coherence between behavioural and physiological responses with age. We discuss these developmental changes in emotion dynamics and behaviour‐physiology associations in the context of functionalist perspectives on emotion.\n\n\nSummary\n\nThis study employs momentary recordings of emotional expressions, regulatory behaviours, and physiological arousal, focusing on two key aspects: within‐task context and temporal dynamics.\nAs they grow, infants show stronger negative reactions and greater regulatory efforts during stress but calmer reactions and reduced use of regulation once stressor ends.\nTemporal associations between emotional behaviours and physiology (heart rate) increased from 6 to 12 months, suggesting stronger emotional coherence with age in infancy.\nTogether, these patterns may reflect improved emotional flexibility, with older infants showing more context‐appropriate emotion responses.\n\n\n"]