Heart rate and skin conductance in four-year-old children with aggressive behavior

Abstract

Autonomic underarousal, indicated by low heart rate (HR) and skin conductance level (SCL), is related to childhood aggression. However, results are inconsistent in preschoolers. We assessed HR, SCL, heart rate reactivity and skin conductance reactivity in four-year-old children. Comparisons were made between children with a high level and with a low level of aggressive behavior according to the Child Behavior Checklist 1½–5 as well as between children who were diagnosed with Oppositional Defiant Disorder or Conduct Disorder (ODD/CD) and children with a low level of aggression. Preschool children with a high level of aggressive behavior showed lower SCL and SCR and children with ODD/CD showed lower SCL. In contrast, we did not find lower HR and HRR in preschool children with a high level of aggressive behavior or ODD/CD. Thus, results suggest that decreased SCL, but not HR, is a characteristic of preschool children with aggressive behavior or ODD/CD.

Introduction

Autonomic underarousal is one of the neurobiological correlates of aggressive behavior (Raine, 2002), i.e., behavior deliberately aimed at harming people (Parke and Slaby, 1983). Heart rate (HR) and skin conductance level (SCL) have been studied as indicators of underarousal in relation to aggressive behavior, both during baseline and in reaction to stimuli. However, as pointed out by Lorber (2004), in empirical studies in this field, children have been defined not only in terms of aggressive behavior, but also in terms of antisocial behavior, delinquent behavior, psychopathic traits and conduct problems.

Results of meta-analyses conducted by Raine (1996) and Ortiz and Raine (2004) indicate that low resting HR is one of the best replicated biological markers of antisocial and aggressive behavior in children and adolescents. Effect sizes in the meta-analyses were calculated using Cohen's d: .20 indicates a small effect, .50 a medium effect and .80 a large effect size (Cohen, 1992). Effect sizes were medium (d = −.56) for HR during rest in children and adolescents with antisocial behavior in the first meta-analysis and in the meta-analysis of 2004 (d = −.44). For heart rate reactivity (HRR), Ortiz and Raine reported a large effect size (d = −.76). In another recent meta-analysis in which children, adolescents and adults were analyzed separately, Lorber (2004) found that conduct problems in children were associated with low resting HR (d = −.34), but greater HRR (d = .20). In this meta-analysis, it was also found that children with conduct problems had lower SCL (d = −.30) and skin conductance reactivity (SCR; d = −.46) (Lorber, 2004). In conclusion, these meta-analyses provide evidence that low resting HR and SCL, as well as SCR, are robust correlates of antisocial behavior in children, but there is less agreement regarding HRR.

As it appears from a large number of studies, there is evidence for autonomic underarousal in elementary school children and adolescents with aggressive or antisocial behavior. Yet, few studies regarding autonomic nervous system (ANS) functioning have been conducted in preschoolers and results are inconsistent. Some longitudinal studies starting in early childhood have been conducted to assess HR and SC as predictors of aggressive or antisocial behavior. In the Mauritius study, it was found that nine-year-old boys, who had been characterized by their teachers as indulging in fighting, had significantly lower SCR at age 3 than their non-fighting peers (Venables, 1989). Furthermore, aggression at age 11 was predicted by low resting HR at age 3 (Raine et al., 1997). However, results from another study revealed that low HR in infancy and toddlerhood (measured at 14, 20, 24 and 36 months) did not predict externalizing behavior problems at age 7 (Van Hulle et al., 2000). From these studies, it remains unclear at what age underarousal in children with aggressive behavior arises. Besides these longitudinal studies, some cross-sectional studies have investigated HR and SC as correlates of aggressive or antisocial behavior. A recent study demonstrated that preschool children with Oppositional Defiant Disorder (ODD) showed less spontaneous skin conductance fluctuations and lengthened cardiac pre-ejection periods (Crowell et al., 2006). In a study in two-year-old children with externalizing behavior, no differences were found on HR (Calkins and Dedmon, 2000). Assuming that aggression and fearlessness are associated (Raine, 1993), the study conducted by Fowles et al. (2000) should be mentioned; fearless four-year-old children showed lower SCR than children who were fearful.

Few studies in preschoolers have been conducted and most studies included only one indicator of underarousal. On top of that, as mentioned earlier, children have been defined from different behavioral perspectives. Therefore, aims of the present study were to assess four indicators of underarousal (HR, SCL, HRR and SCR) in a sample of preschool children defined both from the perspective of aggressive behavior and from the perspective of syndromes of disruptive behaviors, i.e., the DSM-IV-TR categories ODD and Conduct Disorder (CD) (APA, 2000). ODD and CD are clusters of disruptive behavior or clinical syndromes consisting of a large variety of inappropriate behaviors, whereas aggressive behavior is more homogeneous. ODD and CD are being increasingly used to define children in ANS studies (e.g., Beauchaine et al., 2008, Crowell et al., 2006). Moreover, since aggressive behavior is frequently observed in children with ADHD (Connor et al., 2002), we also included children diagnosed with ADHD. To our knowledge, no studies have been conducted regarding ANS functioning in an ADHD-only group. Therefore, we did not formulate specific hypotheses regarding ADHD. Finally, because of a high comorbidity of ODD/CD and ADHD (Angold et al., 1999), a comorbid group was also included. Based on the literature about ANS functioning in school-aged children, we hypothesized that (1) children with a high level of aggressive behavior and (2) children with ODD/CD with or without ADHD, show lower basal levels of HR and SC, and decreased HR and SC responses when compared to children with a low level of aggressive behavior.