The relationship between heart rate and antisocial
behavior in male juvenile delinquents
Masterscriptie Forensische Orthopedagogiek Graduate School of Child Development and Education
Universiteit van Amsterdam Naam: M.R. Bongaards Studentnummer: 10014055 Begeleiders: Dr. T. L. van Zuijen en Dr. A. L. van den Akker
Amsterdam Juli 2015 Table of Contents Abstract 3 Introduction 4 Method 9 Participants 9 Measures 9 Procedure 12 Data-analyses 13 Results 14
Statistical treatment of data 14
Bivariate correlations 14
Mediation analyses 16
Discussion 20
Abstract
A substantial number of studies show a relationship between a low resting heart and antisocial behavior, but this relationship has not been studied in incarcerated juvenile delinquents and the underlying mechanism remains largely unknown. This study addressed these limitations by examining the relationship between heart rate and antisocial behavior in 280 male incarcerated juvenile delinquents and by testing sensation seeking theory, self-control, and empathy as mediators for the low heart rate – antisocial behavior relationship. Heart rate was measured during rest and stress conditions and antisocial behavior was measured using reactive and proactive aggression. Moreover, participants completed self-report questionnaires measuring sensation seeking, empathy, and self-control. Stress heart rate was negative associated with reactive aggression. Sensation seeking, empathy, and self-control did not moderate this relationship. Suggestions for further research to investigate the heart rate – antisocial behavior relationship and the underlying mechanism are given.
Introduction
The focus of this thesis lies on the underlying mechanism between a low heart rate and antisocial behavior of juveniles. To date, it’s known that antisocial adolescents have a lower heart rate, but this relationship has only been investigated in community samples and a valid explanation for this relationship is missing. This thesis investigates whether a lower heart rate – antisocial behavior relationship can be found in male incarcerated juvenile delinquents and whether the sensation seeking theory (Zuckerman, 1979) is a possible explanation for the relationship.
Stress is a (physiological) reaction to a threatening condition or stimulus. The autonomic nervous system regulates stress reactions and is composed of two subsystems: the sympathetic nervous system and the parasympathetic nervous system (Diamond & Cribbet, 2013). In case of danger or threatening stimuli the sympathetic nervous system acts in a fight or flight response. The fight or flight response is associated with increased heart rate, increased respiration rate, blood pressure, cardiac output, and a higher skin conductance (Scarpa, Haden, & Tanaka, 2010). A body cannot sustain the fight or flight state for long periods of time. Therefore, the parasympathetic nervous system is responsible for the restorative processes after a fight or flight reaction. It decreases the heart rate and blood pressure, and returns the body back to its baseline (Muñoz & Anastassiou-Hadjiucharalambous, 2011).
Antisocial adolescents are less sensitive to some forms of stress (Van Goozen & Fairchild, 2008). Popma et al. (2006) found that the activity of the autonomic nervous system as a response to stress was lower for antisocial adolescents than the control group. Antisocial adolescents showed a significantly decreased response in autonomic nervous system activity when they had to perform a public speaking task. Because they are less sensitive to adverse effects of stress antisocial adolescents place themselves in risky or dangerous situations more frequently than other adolescents (Van Goozen & Fairchild, 2008).
A possible explanation for why antisocial adolescents are less sensitive to stress is because their autonomic nervous system works differently (Raine, 1993; Zuckerman, 1979). The (re)activity of the autonomic nervous system can be measured using heart rate, skin conductance, and cortisol (stress hormone). Research shows that a low resting heart rate is the best-replicated biological correlate of antisocial behavior in children and adolescents (Lorber, 2004; Ortiz & Raine, 2004). Ortiz and Raine (2004) conducted a meta-analysis to assess whether antisocial children and adolescents were characterized by low heart rate. A moderate effect was found for the relationship between low resting heart rate and antisocial behavior. A number of alternative explanations were ruled out since the relationship was not moderated by gender, use of psychiatric or normal control group, method of assessing heart rate, concurrent versus prospective research design, source of subjects, recruitment, source of behavioral data, mean age, or year of publication.
It must be noted that not all research finds the relationship between low heart rate at rest and antisocial behavior (Portnoy & Farrington, 2015; Ortiz & Raine, 2004). For example Sijtsema et al. (2010) did not find an association between girls’ heart rate and antisocial behavior. However, they did find this relationship for boys and antisocial behavior. A possible
explanation for not finding a relationship between girls’ heart rate and antisocial behavior is that antisocial behavior of girls is more driven by social-environmental influences and antisocial behavior of boys is more driven by heritable biological markers for antisocial behavior (Beauchaine, Hong, & Marsch, 2008). Portnoy and Farrington (2015) also found a significant effect for low resting heart rate and antisocial behavior in their meta-analysis. However, most of the included studies, which did not find a relationship between resting heart rate and antisocial behavior were unpublished and were retrieved through personal contact. Therefore, there might be a publication bias towards publishing articles on confirming low heart rate – antisocial behavior relationship.
One possible reason why some studies do not find the relationship between heart rate and antisocial behavior is the heterogeneity of the antisocial behavior construct (Lorber, 2004). Aggression, psychopathy, conduct problems, and antisocial personality characteristics are different forms of antisocial behavior. In some studies these constructs are measured together as antisocial behavior (Armstrong, Keller, Franklink, & MacMillan, 2009). In other studies the relationship between heart rate and a specific form of antisocial behavior is investigated (Popma et al., 2006; Portnoy et al., 2014). Lorber (2004) conducted a meta-analysis to investigate the relations of heart rate with aggression, psychopathy, and conduct problems. Analyses showed that resting heart rate was related to aggression and conduct problems and not to psychopathy.
Not only can antisocial behavior be divided in aggression and other constructs, also aggression is multifaceted. Dodge and Coie (1987) distinguish two forms of aggression: reactive and proactive aggression. According to Dodge and Coie reactive aggression is a hostile reaction to provocation or threat and is characterized by impulsive responses and
negative affect. On the contrary, proactive aggression is non-provoked aversive behavior to influence others. It is focused on gaining resources or it is used to intimidate or dominate other persons. Portnoy et al. (2014) found that a lower heart rate was associated with both reactive and proactive aggression. Therefore, reactive and proactive aggression may be used as antisocial behavior measures to examine the heart rate – antisocial behavior relationship.
Albeit the substantial evidence for the low heart rate – aggression relationship there is a limited understanding of the underlying mechanism. A possible explanation for the relationship between lower heart rate and antisocial behavior is the sensation seeking theory (Zuckerman, 1979). The sensation seeking theory argues that some people have an unpleasant physiological state. They are under aroused, which is presumably marked by a low heart rate. To gain a more pleasant arousal level they seek sensation, for example by displaying antisocial behavior. Portnoy et al. (2014) found that impulsive sensation seeking mediated the relationship between heart rate and both aggression and nonviolent delinquency in boys. Girls were not included in the sample. Sijtsema et al. (2010) also found that sensation seeking mediated the relationship between heart rate and aggression and rule breaking. As noted before they didn’t find this relationship for girls.
From the sensation seeking theory’s point of view it is expected that the autonomic nervous system of antisocial adolescents shows the same reaction to stress as their non-antisocial peers. The neurobiology of the sensation seeking theory holds that activity in a number of physiological arousal systems should be reduced under both resting conditions and during normal task performance. It is expected that antisocial individuals in resting state have lower baseline cortisol secretion, lower heart rate, lower skin conductance level, and more slow-wave EEG activity. However, when they are exposed to stress or stimuli involving threat the activity of the autonomic nervous system will by itself increase normally, but
because they have a lower baseline their levels will not reach the same height (Van Goozen & Fairchild, 2008).
Not only could sensation seeking mediate the relationship between a low heart rate and antisocial behavior. Other mediators were proposed by the author. One possible mediator is empathy. Empathy is a person’s ability to understand what another person is feeling and thinking (Van Langen, Wissink, Van Vught, Van der Stouwe, & Stams, 2014). Empathy as mediator between a lower heart rate and antisocial behavior has not been studied before, but theoretical evidence exists for this relationship. The limbic system of the brain is responsible for processing and regulating emotions, including empathy. The physiological state of the body plays an important role in emotions, feelings, and affective experiences (Fukushima, Terasawa, & Umeda, 2010). A lower heart rate could be an indication of an under aroused limbic system. Evidence suggests that under aroused individuals may display dysfunction in empathy (Shirtcliff et al., 2009). Empathy deficits are correlated with higher levels of offending, including aggression (Jolliffe & Farrington, 2004). Therefore, empathy could moderate the relationship between a low heart rate and antisocial behavior.
Another possible moderator is self-control. Self-control is the ability to inhibit inappropriate emotions, desires, and behavior and replace them with suitable ones (Casey, 2015). Research shows a relationship between periodic modulation of heart rate and activation in the prefrontal cortex (Mukhin, Yakovlev, & Klimenko, 2013). The prefrontal cortex plays a crucial role in the execution of self-control (Hare, Camerer, & Rangel, 2009). A lower heart rate could be an indication of under arousal in the prefrontal cortex, whereby individuals with a lower heart rate have more problems with self-control. Individuals who lack in self-control tend to be more aggressive and show more antisocial behavior (Tangney,
Baumeister, & Boone, 2004). Therefore, self-control is proposed as mediator between a low heart rate and antisocial behavior.
The purpose of current study is to determine the underlying mechanism between a low heart rate and antisocial behavior. The first purpose of this study is to confirm if there is a relationship between low heart rate and antisocial behavior in male incarcerated juvenile delinquents. Heart rate will be measured during a rest condition and a stress condition. Antisocial behavior will be measured by reactive and proactive aggression. It is hypothesized that juvenile delinquents have who a lower heart rate tend to show more antisocial behavior. The second purpose of this study is to explore whether sensation seeking, empathy, and self-control moderate the relationship between a low heart rate and antisocial behavior. It’s hypothesized that sensation seeking, empathy, and self-control moderate the relationship between heart rate and antisocial behavior.
Method Participants
Participants were 283 male juvenile delinquents between the ages of 12 and 23 years (mean = 18.73, SD = 1.62) from five juvenile justice institutions in the Netherlands. The data were collected between the beginning of 2014 and April 2015. Adolescents were included if they were referred to a juvenile justice institution, were between 12 and 24 years old, and were suspected of committing an offense or having already been convicted. Exclusion criteria for participation were insufficient command of the Dutch language, inability to understand the instructions and questionnaires, and unwilling or unable to sign informed consent. The ethnicity breakdown of the sample was as follows: 78.3% Dutch, 6.8% African, 5.1% Antillean, 5.1% European, and 4.7% Asian or South-American.
Physiological assessment. Heart rate was measured using the VU- Ambulatory Monitoring System (VU-AMS; Klaver, De Geus, & De Vries, 1994). A total of seven disposable electrodes were placed on the body, however, only three of the seven electrodes are needed to measure heart rate. The first electrode was placed under the clavicle, 4 centimeters to the right of the sternum. The second electrode was placed on the right chest between the two lowest ribs. The third electrode was placed on the left chest, under the nipple at the height of the xiphiod. The electrodes were connected with lead wires to the VU-AMS. Heart rate (in beats per minute; bpm) was extracted from the electrocardiogram (ECG). Heart rate was computed by averaging interbeat intervals throughout each task and converting values to beats per minute.
Aggression. Aggression was measured using the reactive-proactive questionnaire (RPQ; Raine et al., 2006). The Dutch RPQ was used and is validated for the groups non-offender participants, criminal non-offenders, youngsters (age 6-18), and adults (age above 18) (Cima, Raine, Meesters & Popma, 2013). The RPQ is a 23-item behavior rating scale developed to asses children and adolescents aggression. It distinguishes between reactive and proactive aggression, with 11 items measuring reactive aggression (e.g., “reacted angrily when provoked by other”) and 12 items measuring proactive aggression (e.g., “hurts others to win a game”). These items were rated by the participant on a three-point scale (0 = never, 1 = sometimes, 2 = often). Scales can be calculated for proactive aggression (maximum score: 24) and reactive aggression (maximum score: 22). Higher scores on the scales means higher levels of whatever dimension that scale represents. In the current study both subscales showed good reliability (α = .86).
Sensation seeking. To measure sensation seeking the Dutch subscale sensation seeking of the Substance Use Risk Profile Scale (SURPS; Woicik, Stewart, Pihl, & Conrod,
2009) was used. The SURPS is designed as a brief assessment tool to measure four distinct personality dimensions (Axiety Sensitivity, Hopelessness, Sensation Seeking, and Impulsivity). These personality dimensions are thought to be linked to a vulnerability for a particular reinforcement pattern that may lead to substance use. The SURPS is validated for use with a Dutch adolescent population (Malmberg et al., 2010). The 23-item questionnaire was used, because the 28-item version does not provide any additional predictive power over the 23-items version (Woicik et al., 2009), making the 23-item version preferable as it takes less time for the participant to complete. Examples of items are ‘I would like to skydive or I like doing things that frighten me a little’. The items can be answered on a 4-point Likers scale (1 = strongly disagree, 2 = disagree, 3 = agree, 4 = strongly agree). A higher score on the subscale sensation seeking means a higher level of personality dimension sensation seeking. In the current study sensation seeking subscale of the SURPS showed acceptable reliability (α = .63).
Self-control. To measure self-control the Dutch version of the Brief Self-Control Scale (BSCS; Maloney, Grawitch, & Barber, 2012) was used. The BSCS was developed to assess dispositional self-control as it is conceptualized by contemporary theoretical perspectives and has shown good reliability and validity (Malouf et al., 2013). The questionnaire consists 13-items which can be answered on a five-point Likert scale (1 = not at all like me, 5 = very much like me). Examples of items are ‘I can resist temptations or I am lazy’. The subject answers the items based on the past six months. The possible range for control is a total score between 13, not at all controlled, to 65, extremely self-controlled. In the current study the BSCS showed good reliability (α = .78).
Empathy. To measure participants empathy the Index of Empathy for Children and Adolescents (IECA; Bryant, 1982) was used. The IECA is a 22-item self-report questionnaire
and assesses participant’s empathic behavior and feelings. For the present study, the items from the empathic sadness subscale were translated into Dutch, which gives an indication of reactive empathy as it relates to feelings of sadness (De Wied et al., 2007). Examples of items are ‘Seeing a boy/girl who is crying makes me feel like crying or I get upset when I see a boy/girl begin hurt.’ Each item is rated on a 2-point scale (0 = disagree, 1 = agree). The possible range for the empathic sadness subscale is a total score between 0 to 14. A higher score is indicative of higher reactive empathy related to sadness. In the current study the sadness subscale of the IECA showed good reliability (α = .83).
Countdown task. The countdown task is a stressor task. The countdown task is composed of three signaled (trials 1, 3, 5) and two un-signaled (trials 2 and 4) white noise bursts. Every noise is a 1 second white noise burst of 85 decibel and the mean intertrial interval was 45 seconds. Participants were instructed via de screen of the laptop that a loud noise would follow from a countdown from twelve to zero on the screen. No numeric countdown was shown prior to the two un-signaled noises participants. Participants were not aware of the number of noises and the alternating (signaled, signaled, signaled, un-signaled, signaled) nature of the trials
Procedure
The study was approved by the Medical Ethics Committee of the VU University Medical Center Amsterdam, and all participants as well the parents of minors gave written informed consent. After obtaining the informed consent from the participants an appointment for the session was scheduled. Adolescents were assessed in one session of approximately 1 and a half hour. For some participants the session was too long and for them the session was divided in two parts. At the start of the session the participant was asked if he/she had eaten, exercised, or smoked the hour before the session, because this could influence the
neurophysiological data. If they hadn’t smoked, eaten, or exercised the procedure continued, when they had smoked, eaten, or exercised the information was written down and participants were given a bottle of water to drink to reduce the influence of the food and smoking on the neurophysiological measures. After the check the VU-AMS was attached. Electrodes were placed on the body (as described in measures). Next, the participant was asked to sit quietly in a chair and the signals were checked. While checking the signals the researcher explained to the participant what he saw on the screen and what would happen if he moved too much. If the signals were received well the neurophysiological measures started and the participant was asked to fill in questionnaires on a laptop to get used to the electrodes and wires. When the participant had filled in the questionnaires, the neurophysiological measures were checked one last time before the start of the countdown task. If the signals were still received well, the participant was provided with a gently reminder to sit still. Prior to the countdown task came a 5 minute resting period. Participants were instructed to relax by watching a 5 minute film clip showing swimming fish, while hearing relaxation music. If they could not relax by watching the film clip, participants were told to look at the walls in the room.
Participants who completed the whole session were compensated for their time with a €5 call credit or a shower product. Because this research was part of a larger study, the participants also completed other measures, tasks, and questionnaires in the same session that are not reported here.
Data-analyses
To investigate the relationship between heart rate variables and aggression bivariate correlations were calculated. The causal steps strategy (Baron & Kenny, 1986) is a very widely used method for mediation analyses, but this approach has big flaws. It has the lowest power among existing mediation analyses and it’s questionable whether the causal steps
approach tests a mediation effect (Hayes, 2009). Therefore a bootstrapping method (Preacher and Hayes, 2008) was used to investigate if sensation seeking, self-control, and empathy mediate the relationship between heart rate and aggression. Bootstrapping depends on repeatedly sampling data and estimates the mediation effect in each resampled data set. By replicating the bootstrapping process a thousand times an approximation of the sampling distribution can be calculated. Which can be used to calculate confidence intervals of the mediation effect. When the confidence interval does not contain zero a mediation effect is detected.
To help researchers Preacher and Hayes (2008) wrote a macro for SPSS which gives SPSS the commands to bootstrap the data. Next, it’s only necessary to execute INDIRECT-formula in which the independent, dependent, and moderating variables are designated. After this command SPSS’ output shows the regression coefficient a, b, c, and c’ of the mediation model (see figure 1) together with relevant statistic values and the confidence interval of the mediation effect.
Figure 1. Mediation Model
Independent Variable Dependent Variable Mediator A path B path Independent Variable Dependent Variable C path
Results Statistical treatment of data
Data were checked for univariate outliers. Observed variables were considered to be an outlier when the z-score was lower than -2.807 or higher than 2.807 (Cousineau & Chartier, 2010). Three outliers were found for proactive aggression and two outliers for self-control. Next, data were checked for multivariate outliers. No multivariate outliers were found. Because the skewness ratio (SR = 5.29) for proactive aggression exceed a score greater than 3 the three cases were deleted. The two outliers for self-control were not deleted, because the skewness ratio did not exceed a score greater than 3 (SR = -.65). Further statistical analyses were conducted with 280 participants (N = 280).
Bivariate associations
The assumption that a low resting heart rate was associated with higher levels of reactive and proactive aggression was not met. The resting heart rate did not correlate with any form of aggression. Neither did the resting heart rate correlate with sensation seeking, self-control, or empathy. Given the high intercorrelations (r < .93) between the mean responses of the signalled stressor and mean responses of the unsignalled stressor a new variable heart rate stress was created. It consisted the mean of the five responses to the white noise burst. Because the first assumption was not met. further statistical analyses were only conducted with the stress heart rate, but it must be noted that resting heart rate and stress heart rate also correlated very high (r = .95).
Bivariate correlations among the observed variables are shown in table 1. Pearson’s correlation (r) was used to calculate the bivariate associations. Stress heart rate was negatively associated with reactive aggression, but did not correlate with proactive aggression. Neither was stress heart rate associated with sensation seeking, self-control, or empathy. Sensation seeking was positively correlated with reactive and proactive aggression. Empathy correlated with proactive, but was not associated with reactive aggression. Self-control was negatively associated with reactive and proactive aggression. Moreover, sensation seeking was negatively correlated with self-control and did not correlate with empathy. Self-control and empathy were positively correlated. Furthermore, reactive and proactive were positively correlated.
Table 1
Bivariate Correlations Among Observed Study Variables, Together with Means and SDs
Variable 1 2 3 4 5 6
1. Stress heart rate
2. Reactive aggression -.13*
3. Proactive aggression -.01 .72**
4. Sensation seeking -.10 .36** .29** .
*p < .05; **p < 0.01
Mediation analyses
Bivariate associations among variables are not a necessary condition to test mediation (Hayes, 2009). Therefore, all hypothesized mediators were tested between stress heart rate and proactive and reactive aggression. The mediation analyses betweenheart rate, the hypothesized mediators, and proactive aggression did not add extra knowledge other than the knowledge that can be derived from the bivariate correlations. Hence, only the mediation analyses between stress heart rate, hypothesized mediators, and reactive aggression are reported.
In advance, it must be noted that discrepancies emerged between the coefficients of the bivariate correlations and the coefficients calculated with the bootstrapping method. The bootstrapping method uses list wise deletion based on all variables in the model. For example, when calculating the effect of the independent variable on the mediator, not only are de missing cases of the independent variable and mediator thrown out, but also the cases with missing data on the dependent variable. This is standard procedure in the assessment of models like this. Therefore, discrepancies could emerge because of the different sample sizes that are used. Sample sizes of the bootstrapping method and bivariate correlations are shown in table 2.
Table 2
Sample Sizes used in the Bootstrapping Method and Sample Sizes Range of the Bivariate Correlations
6. Empathy -.06 -.11 -.18** .03 .29**
Mean 70.22 9.64 5.60 15.12 45.05 16.14
First, sensation seeking was tested as mediator between stress heart rate and reactive aggression (See figure 2). The bootstrapping method showed that the mediation effect was not significant (-0.0174 - ; 95% CI between -0.0424 and .0028). Stress heart rate was not associated with sensation seeking (β = -.04, t = -1.52, p = .13), but sensation seeking was positively associated reactive aggression (β = .42, t = 5.76, p = .00). In this model, the stress heart rate and reactive aggression were not significantly associated anymore (β = -.05, t = -1.54, p = .12) and remained insignificant after controlling for sensation seeking (β = -.03, t = -1.07, p = .29). Indicating that juvenile delinquents who are more sensation seeking show more reactive aggression. There is, however, no evidence for sensation seeking as mediator between heart rate and reactive aggression.
Figure 2. Mediation Analysis between Stress Heart Rate, Sensation Seeking, and Reactive Aggression
N Bootstrapping method N Bivariate correlation
Sensation Seeking 235 235 - 237
Empathy 243 243 - 267
Self-control 245 245 - 267
Secondly, empathy was tested as mediator between stress heart rate and reactive aggression (See figure 3). The bootstrapping method showed that the mediation effect was not significant (0.0025; 95% CI between -0.0022 and .0128). Stress heart rate was not associated with empathy (β = -.03, t = -.84, p = .40) and empathy was not associated with reactive aggression (β = -.10, t = -1.46, p = .15). The total effect of stress heart rate on reactive aggression was significant (β < -.06, t = -1.97, p < .05) and after controlling for empathy this relationship remained significant (β < -.06, t = -2.05, p = .04). These results indicate that empathy does not moderate the relationship between stress heart rate and reactive aggression and empathy has no predictive value for reactive aggression.
Figure 3. Mediation Analysis between Stress Heart rate, Empathy, and Reactive Aggression
Stress heart rate Reactive aggression
Sensation seeking
β = -.04 β = .42**
Third, self-control was tested as mediator between stress heart rate and reactive aggression (Figure 4). The bootstrapping method showed that the mediation effect was not significant (0.0023; 95% CI between -0.0280 and .0320). Stress heart rate was not associated with self-control (β = -.01, t = -.14, p = .89), but self-control was negatively associated with reactive aggression (β = -.30, t = -10.15, p = .00). The total effect of stress heart rate on reactive aggression was significant (β < -.06, t = -1.94, p < .05) and after controlling for self-control this relationship remained significant (β < -.06, t = -2.40, p = .02). These results indicate that self-control does not moderate the relationship between stress heart rate and reactive aggression. However, these results show that both self-control and stress heart rate have predictable value for the reactive aggression.
Stress heart rate Reactive aggression
Empathy
β = - .03 β = -.10
Figure 4. Mediation Analysis between Stress Heart Rate, Self-control, and Reactive Aggres sion
Discussion
The purpose of this study was to confirm the relationship between a low heart rate and aggression in male juvenile delinquents and to examine the underlying mechanism between a low heart rate and aggression. The relationship between a low resting heart rate and antisocial behavior in male juvenile delinquents could not be confirmed, but a lower heart rate during a stressor was associated with higher levels of reactive aggression. Sensation seeking, empathy, and self-control did not mediate the relationship between a lower stress heart rate and reactive aggression. However, juvenile delinquents who are more sensation seeking are also more aggressive and juvenile delinquents who have lower self-control are also more aggressive. Moreover, lower empathy correlated with higher levels of proactive aggression.
This study is the first to investigate the relationship between heart rate and aggression
Stress heart rate Reactive aggression
Self-control
β = -.01 β = -.30**
Stress heart rate Reactive aggression
in male incarcerated juvenile delinquents and one of the first studies to explore the underlying mechanism. Although meta-analyses (Ortiz & Raine, 2004; Portnoy & Farrington, 2015) demonstrated the relationship between a low resting heart rate and antisocial behavior in children, adolescents, and adults, this relationship was not found for male juvenile delinquents. Several explanations exists for not finding the hypothesized relationship. First, it is questionable if the resting heart rate genuinely was measured during a rest condition. Living in an juvenile justice institution can emanate severe stress due to confinement and group climate. During confinement juvenile delinquents experience isolation, boredom, and bullying by other inmates which are potential stressors (Greve, 2001). Also the group climate is a potential stressor, particularly a repressive climate, because often group workers and juvenile delinquent lack one another’s trust (Van der Helm, Klapwijk, Stams, & Van der Laan, 2009). Further, subjects could have experienced stress during the rest condition due to the novelty of the experimental situation (Portnoy et al., 2014). For almost all participants it was the first time their heart rate was measured using electronic device. They had never seen the electrodes and wires before. Although, participants had time to get used to the device, the setting itself could derive stress by not knowing what exactly was going to happen.
Another explanation for not finding the low resting heart – antisocial behavior relationship is the homogeneity of the subjects of the current study is. The main interest of this study was if a low heart rate – antisocial behavior relationship could be found in incarcerated juvenile delinquents. Cima et al. (2013) showed that Dutch juvenile delinquents and adult delinquents from prison were significantly more reactive and proactive aggressive than non-offenders and adult non-offenders. The reactive and proactive mean scores of the subjects of the current study did not differ very much with the mean scores of the juvenile delinquents and adult delinquents from prison participating in study of Cima et al. (2013).
Therefore, it could be that by only involving juvenile delinquents as subjects the population already was more aggressive and due to lower variability in the sample size correlations disappeared.
Albeit, the relationship between a low resting heart rate and aggression was not found, a negative relationship was found between stress heart rate and reactive aggression. This finding is consistent with Ortiz and Raine’s (2004) meta-analysis. They found a large effect size for the negative relationship between stress heart rate and antisocial behavior. Lorber (2004) did not find a relationship between task heart rate and aggression. Still, taking a closer look, task heart rate was divided in tasks during a nonnegative stimuli and negative stimuli. A negative relation emerged between task heart rate using adverse stimuli and aggression and for stimuli of negative valence. These corroborating findings indicate that juvenile delinquents with a lower heart rate during stress are more aggressive.
Although it was found that a lower stress heart rate was associated with higher forms of reactive aggression, this result must be interpreted by the limitation that the resting heart rate and stress heart rate correlated very strongly. Other studies (Choy et al., 2015; Portnoy et al., 2014) showed the same problem. Both studies created one heart rate variable out of the different heart rate measures during rest, task, and stress conditions to overcome this problem. Because the resting heart slightly weakened the relationship between heart rate and aggression the decision was made to conduct further analyses with only stress heart rate.
With regard to the second purpose of this study, sensation seeking, empathy, and self-control did not mediate the stress heart rate – reactive aggression relationship. To date, this is the first study to investigate de role of sensation seeking between stress heart rate and aggression. Portnoy et al. (2014) did find sensation seeking as mediator for the heart rate – aggression relationship. However, they used impulsive sensation seeking as mediator. On
theoretical grounds sensation seeking and impulsivity are two different constructs. Sensation seeking is a more planned need for exciting and risky experiences (Zuckerman, 1979). Impulsivity, on the contrary, contains more hasty decision-making and carelessness (Schalling, 1978). Research shows mixed results. Some researchers found sensation seeking and impulsivity loading onto a single factor (Zuckerman, Kuhlman, Joireman, Teta, & Kraft, 1993), while others found sensation seeking and impulsivity as different psychometric measures (Whiteside & Lynam, 2001). By comparing the instruments used in Portnoy et al. (2014) and in the current study it is concluded that they measured a different construct.
Although sensation seeking, empathy, and self-control did not mediate the heart rate – antisocial behavior relationship, on behavioral level juvenile delinquents who were more sensation seeking were also more reactive and proactive aggressive. This finding is in line with other research. A meta-analysis conducted by Wilson and Scarpa (2011) found a positive relationship between sensation seeking and aggression. Raine et al. (2006) found sensation seeking positively correlated with reactive and proactive aggression in adolescent boys. Their coefficients were smaller than the coefficients of the current study. This could indicate that the effect of sensation seeking on reactive and proactive aggression is stronger in juvenile delinquents than their nondelinquent peers.
Self-control was strongly associated with reactive and proactive aggression. According to Gottfredson and Hirschi’s (1990) general theory of crime, self-control is the most important personal trait in explaining antisocial behavior. Tangney et al., (2004) found that people with low self-control showed more aggressive responses to anger-evoking situations compared to people with higher levels of self-control. Furthermore, a meta-analyses showed that lower self-control is related to aggression (De Ridder, Lensvelt-Mulders, Finkenauer, Stok, & Baumeister, 2012). No other study was found to have
examined the relationship between self-control and reactive and proactive aggression. Even though the relationship between self-control and reactive and proactive aggression has not been studied before, impulsivity, the propensity to low self-control (Carrasco, Barker, Tremblay, & Vitaro, 2006), and aggression have been studied a lot. Connor, Steingard, Cunningham, Anderson, and Melloni (2004) found that more impulsivity was associated with more proactive and reactive aggression in children and adolescents. However, Raine et al. (2006) showed only a positive relationship between reactive aggression and impulsivity, but not with proactive aggression.
Empathy was only negative associated with proactive aggression. Proactive aggression is an instrumental form of aggression to gain resources and is associated with the expectancy to feel good about the outcome of criminal activity (Smithmyer, Hubbard, & Simons, 2000). Positive feelings about criminal activities could be an indication of empathy deficits, because some argue that affective empathy discourages proactive aggression (Lovett & Sheffield, 2007). Moreover, research showed that adults who displayed instrumental aggression lacked guilt and empathy for their victims (Cornell, Warren, Hawk, & Stafford, 1996). Not all research finds that lower empathy is only associated with proactive aggression. Mayberry and Espelage (2007) found that both proactive and reactive aggression were associated with lower empathy in middle school children. Empathy deficits seems to be a stable trait during childhood and adolescence and could designate youth with specific forms of aggression (Frick & White, 2008).
Suggestions for further research are reinvestigating the heart rate – antisocial behavior relationship in juvenile delinquents with the use of other rest and stress conditions. Device that is less time consuming to attach and looks more familiar to subjects is recommended. For example, measuring heart rate using wrist pressure cuffs, which has shown to be a reliable
method to measure heart rate (Cauffman, Steinberg, & Piquero, 2005). As stress condition the public speaking task as stressor is proposed. Research shows that the public speaking task is an effective stressor in children and adults (Dickerson & Kemeny, 2004; Kudielka, Buske-Kirschbaum, Hellhammer, & Buske-Kirschbaum, 2004) and meets the most important criteria of an anxiety-arousing condition (Popma et al., 2006). In addition, impulsivity is recommended to investigate as mediator between lower heart rate and antisocial behavior. Portnoy et al., (2014) found that impulsive sensation seeking mediated the lower heart rate – antisocial behavior relationship and in the current study a strong negative relationship was found between self-control and aggression.
Another suggestion for further research is to include antisocial and non-antisocial populations to investigate if the heart rate – antisocial behavior exists among a broader population. Cauffman et al. (2005), for example, did find a relationship between a resting heart rate and antisocial behavior/aggression using adolescents attending public high schools and incarcerated adolescents as subjects. Moreover, the current study was a smaller part of larger research. Other physiological (skin conductance, cortisol) measures, questionnaires, and tasks have been conducted, which makes it possible to compare physiological measures of juvenile delinquents in further research, and also to investigate the relationship between psychological measures and other forms antisocial behavior.
Taken together, heart rate was not measured right, therefore it was difficult to reach a conclusion if the heart rate – antisocial behavior relationship exists in incarcerated juvenile delinquents. When heart rate is measured better, a relationship between a lower heart rate and antisocial behavior may be found. Likewise sensation seeking, empathy, and self-control may be found to be underlying mechanisms of the lower heart rate – antisocial behavior relationship, because theoretically the proposed underlying mechanisms still make sense. The
sensation seeking theory (Zuckerman, 1979) argues that antisocial individuals are under aroused and therefore are more sensation seeking to gain a more pleasant level of arousal. Moreover, autonomic under arousal could also indicate lower activation in the brain structures executing empathy and self-control, which could lead to more antisocial behavior. Furthermore, the current study recommends to include impulsivity as mediator, because impulsive sensation seeking was found in another study (Portnoy et al., 2014) to mediate between a lower heart rate and aggression, and self-control was strongly associated with aggression in the current study. To conclude, sensation seeking, empathy, and self-control were all positively associated with aggression, which is supported by an evidential body of research.
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