Although low resting heart rate is often cited as “the best-replicated biological correlate” of antisocial behaviour, this association remains unexplained. Several theoretical explanations have been proposed throughout the years, but they have been mostly based on unfounded and questionable assumptions. Amongst these, fearlessness and stimulation-seeking hypotheses have gained ground and become well established in the literature, despite the fact that they have been subjected to scarce empirical verifications yielding mixed results. Therefore, employing situational action theory as a theoretical framework, a new explanatory model of the relationship between low heart rate and crime was developed. It was argued that heart rate cannot be a cause of crime, but only a marker of a cause of crime. Specifically, it was hypothesised that a person’s crime propensity would fully mediate the link between heart rate and crime. In order to test the crime propensity model and compare its efficiency against the fearlessness and stimulation-seeking hypotheses, the Cambridge adolescent behaviour study was conducted. It set out to explore the mechanism underlying the heart rate-crime association in a sample of 487 adolescents recruited from 14 schools in England. Participants completed self-report measures of stimulation-seeking, fearlessness, crime propensity, and criminal behaviour, and their heart rate was measured at rest and in response to a stressor. Crime involvement was significantly associated only with heart rate reactivity, whereas the link with resting heart rate was non-significant. Out of the three models with fearlessness, stimulation-seeking, and crime propensity as potential mediators, only crime propensity fully mediated the link between heart rate reactivity and crime, rendering the direct effect of heart rate reactivity on crime non-significant. Therefore, the empirical results supported the newly developed crime propensity model and highlighted the benefits of replacing the prevalent risk factor approach in (biosocial) criminology with an analytical approach that aims to explain what causes crime and how, and, equally importantly, what cannot cause crime.