Association between low resting heart rate and violent behaviour

I read with interest the article of Murray et al. on the association between low heart rate and violent and non-violent crime found in a Brazilian population.1 According to the authors there are two possible explanations for this observation. The first one is that low resting heart rate reflects low autonomic arousal which would cause a stimulation-seeking behaviour. The second explanation focuses on fearlessness. Committing antisocial and violent acts would require ‘a degree of fearlessness which is indicated by low levels of arousal during mildly stressful psychophysiological tests’. Both theories put the emphasis on resting heart rate as a marker of autonomic nervous system tone. In this contention, it would be important to have information about the cardiovascular reactivity of the subjects participating in this study, which in a clinical setting is currently extrapolated from the so-called white-coat effect.2,3 The white-coat effect is considered a measure of reactivity to stress and is usually calculated from the difference between blood pressure and/or heart rate measured by the doctor in the office and blood pressure and/or heart rate self-measured by the patient out of the office.2,3 However, recent research suggests that the same information can be acquired from the readings obtained during a short stay in the office using an automatic device.4,5 In the Methods section of the Murray et al.’s article, the authors report that two measurements of heart rate were taken, the first at the beginning of the assessment session and the second at the end of the assessment procedures.1 This protocol implies that a relatively long period of time elapsed from the first to the second heart rate measurement (probably half an hour or more), which allowed the subject under investigation to slowly adapt to the medical environment. Actually, recent research suggests that the white-coat effect may be reduced by re-measuring blood pressure and heart rate even over a shorter space of time.6,7 For many subjects, repeat measurements result in a reduction in blood pressure up to 15/16 mmHg, virtually eliminating any white-coat effect.6,7 The same trend has been observed for heart rate, as previously shown in our laboratory.8,9 In a study by Sheppard et al., it was shown that the difference between the first and last measurement obtained within a short period (5 min) was closely correlated with the white-coat effect calculated from the difference between the first office measurement and the mean of the readings obtained at home over a 7-day period.10 Thus, the authors of the present study1 have the potential for calculating a surrogate of the white-coat effect using this difference between the first and the second heart rate measurement. This measure could be used to investigate if people with antisocial behaviour have lower reactivity to stress. We thus encourage the authors of this article to test this hypothesis in their population, which could increase our knowledge of the mechanisms involved in the heart rate-social behaviour association. Another interesting finding of the authors’ study, that merits discussion, is that the association of low heart rate with violent behaviour was weaker in females.1 This is consistent with the finding of a weaker relationship of heart rate with cardiovascular events and mortality in women compared with men, observed in many studies.11,12 This gender-related difference has been attributed to heart rate being less representative of the autonomic nervous system tone in women compared with men. This interpretation can also apply to the present findings, giving further support to the theory that violent behaviour is more common in people with low autonomic arousal.