this study explored two competing hypotheses explaining the association of heart rate variability (hrv) and romantic relationships. hrv refers to the variation between successive heartbeats and is considered to be a noninvasive index of the social engagement system. recent cross-sectional research has shown differences in hrv reactivity (the difference between hrv during the presentation of positive and negative stimuli) between single and newly coupled individuals. two opposing explanations for this association were hypothesized. first, beginning a romantic relationship may decrease hrv reactivity through increased activation of the social engagement system during courting. alternatively, this association could be explained by individuals with initially lower hrv reactivity being more likely to form romantic relationships. in the present study, single female undergraduate 世界杯2022赛程表淘汰赛 were presented with film clips of various valences while having their cardiac activity monitored. these participants then returned for follow-up either when they had begun a romantic relationship, or at the end of the 6-month observation period if they remained uncoupled. moderating influences of body mass index (bmi), self-esteem, attachment, and emotional distress were assessed. results revealed single and coupled participants were comparable in terms of their hrv reactivity at follow-up; neither group showed a significant decline in hrv during the negative film clip. further, hrv was not systematically affected by a change in relationship status from single to newly coupled. however, lower hrv reactivity was predictive of coupling for low bmi women while the reverse was true for high bmi women. this interaction may be the result of differing success rates of various mating strategies for low and high bmi women. results support the hypothesis that hrv reactivity, along with bmi, can predict the formation of romantic relationships.