What are the mechanisms whereby low resting heart rate predisposes to antisocial behavior? Low arousal, stimulation seeking, fearlessness, increased vagal tone/vagal passive coping, reduced noradrenergic functioning, and reduced right hemisphere functioning represent several of the possible processes which, either by themselves or in combination, may predispose a child to aggression. Physiological explanations of the heart rate–antisocial relationship consist of arousal theory, vagal tone, norepinephrine, and right hemisphere dysfunction.
Low arousal. The most obvious and simple physiological explanation is that heart rate is a measure of autonomic arousal, and that low physiological arousal in general is a predisposition to antisocial and criminal behavior (Eysenck, 1987; Raine et al., 1990). In support of this relatively simple theory, which also invokes stimulation-seeking (see below), there is growing evidence that other measures of autonomic and central nervous system functioning, including resting EEG (Raine et al., 1990), skin conductance activity (Fowles, 1993; Raine et al., 1990), and cortisol (McBurnett et al., 1991; Susman& Petersen, 1992; van Goozen et al., 1998) are related to antisocial and conduct disordered behavior in children.
On the other hand there appears to be no unitary arousal system, as intercorrelations between these different measures of arousal are low or even nonexistent in the general population (e.g., Raine et al., 1990). However, it is conceivable that an extreme(antisocial) group within this general population does have low arousal on multiple arousal measures. Some evidence does exist for underarousal on at least two separate physiological measures of arousal in antisocial child and adolescent samples (e.g., van Goozen et al., 1998; Raine et al., 1990).
Stimulation seeking theory. Stimulation-seeking theory argues that low arousal represents an unpleasant physiological state; antisocial individuals seek stimulation in order to increase their arousal levels to an optimal or normal level (Eysenck, 1997; Quay, 1965; Raine et al., 1997a). Antisocial behavior is thus viewed as a form of stimulation-seeking, in that committing a burglary, assault, or robbery could be stimulating for some individuals. In support of this theory, El-Sheik, Ballard, and Cummings (1994) found that preschool boys who chose to watch videotapes depicting intense anger had lower heart rates than controls, and also that low resting heart rate was associated with externalizing problems. This indicates some support for the view that low heart rate characterizes both stimulation-seeking and antisocial behavior. Similarly, resting heart rate at age 3 years has been found to characterize stimulation-seeking behavior at 3 years, as well as aggressive behavior at 11 years (Raine et al., 1997a, 1998a).
Fearlessness theory. Fearlessness theory argues that low levels of arousal during mildly stressful psychophysiological test sessions are markers of low levels of fear (Raine, 1993, 1997). For example, particularly fearless individuals such as bomb disposal experts who have been decorated for their bravery have particularly low heart rate levels and reactivity (Cox, Hallam, O’Connor, & Rachman, 1983; O’Connor, Hallam, & Rachman, 1985), as do British paratroopers decorated in the Falklands war(McMillan & Rachman, 1987). Antisocial and violent behavior (e.g., fights and assaults) requires a degree of fearlessness to execute, and lack of fear of socializing punishments in early childhood would contribute to poor fear conditioning and lack of conscience development (Raine, 1993). Fearlessness theory receives support from the fact that low heart rate also provides the underpinning for a fearless or uninhibited temperament in infancy and childhood (Scarpa, Raine, Venables, &Mednick, 1997; Kagan, 1994).
Vagal tone. Another physiological mechanisms to account for the heart rate–antisocial relationship is increased vagal tone. Raine and Venables (1984) first suggested that the low heart rate recorded in antisocial individuals may be a function of increased vagal tone and reflect a passive coping response to mildly stressful situations (i.e., so-called ‘resting states’ prior to some other experimental manipulation). Data since then have not supported this hypothesis, favoring instead increased parasympathetic activity in antisocial individuals. For example, Mezzacappa et al. (1997) found that antisocial 15-yearold boys were characterized by low resting heart rates, but also found evidence for reduced, not increased, vagal functioning. Furthermore, Pine et al. (1996) found reduced, not increased, vagal tone was associated with aggressive behavior in children. If reduced vagal tone does prove a systematic correlate of antisocial behavior, it will indicate that the resting heart rate–antisocial relationship is driven by particularly strong sympathetic underarousal which is powerful enough to overcompensate for the lack of parasympathetic influences (low vagal tone) which would otherwise be expected to increase heart rate in antisocial children.
Reduced noradrenergic functioning. Strong underarousal of the sympathetic nervous system would support a neurochemical explanation of the heart rate–antisocial relationship based on reduced noradrenergic functioning. The monoamine norepinephrine, found in autonomic nervous system neurons and produced in the locus coeruleus, is centrally involved in attention and vigilance and forms one of the four arousal systems in the brainstem. While studies of peripheral measures of norepinephrine in antisocial children have found weak positive or null effects (Berman, Kavoussi, & Coccaro, 1997), a meta-analytic review found a significant negative effect size of .41 between reduced central (cerebrospinal fluid) measures of norepinephrine and increased antisocial behavior (Raine, 1993). Furthermore, Rogeness et al. (1990a, b) found both reduced heart rate and reduced noradrenaline in conduct disordered children.
Reduced right hemisphere functioning. Poor right hemisphere functioning could underlie the low heart rate–antisocial relationship. The right hemisphere is dominant for the control of autonomic functions, including heart rate (Lane & Jennings, 1995) and both lesion and intracarotid amobarbital studies confirm that reduced heart rate is associated with decreased right hemisphere functioning (Zamrini et al., 1990; Yokoyama, Jennings, Ackles, Hood, & Boller, 1987). In turn, although left hemisphere dysfunction has frequently been implicated in violence and crime (Raine, 1993), poor right hemisphere functioning has also been found in antisocial and violent populations as measured by functional magnetic resonance imaging (fMRI – Raine et al., 2001a), computerized tomography (CT – Hucker et al., 1988), neuropsychological tests (Day & Wong, 1996), spatial IQ measures (Raine, Yaralian, Reynolds, Venables, & Mednick, in press), EEG deficits (Evans & Park, 1997), and event-related potentials (ERPs – Drake, Pakalnis, Brown, & Hietter, 1988). Poor right hemisphere functioning (particularly the anterior regions) has been associated with deficits in the withdrawal system, a system that promotes retreat from aversive and dangerous situations (Davidson, Eckman, Saron, Senulis, & Friesen, 1990; Davidson, 1998). Furthermore, patients with right hemisphere lesions, compared to those with left hemisphere lesions, have been shown to have reduced heart rate and skin conductance responses to films depicting negative emotions (e.g., anger) (Zoccolotti, Caltagirone, Benedetti, & Gainotti, 1986). Reduced right hemisphere functioning and a consequent weaker withdrawal system could make children less averse to dangerous, risky situations that increase the probability of antisocial behavior.
