The effect of childhood trauma and cognitive control on aggression

Master Thesis Clinical Neuropsychology

Faculty of Behavioural and Social Sciences – Leiden University

September, 2019

Student number: 1907344

First Examiner: Ilse Schuitema, Health, Medical and Neuropsychology Unit;

Leiden University

The effect of childhood trauma and cognitive control

on aggression

Content

Abstract

Aggression is found to be a major public health problem and is particularly prevalent among the forensic population. Previous research not only suggests associations between a history of childhood trauma and aggression but also between cognitive control (inhibition) and aggression. However, the causes underlying this relationship have remained relatively unexplored. The main purpose of the current study was to investigate the potential mediating effect of inhibition in the relationship between childhood trauma and aggression. More insight into this relationship can contribute to the development of effective interventions ultimately resulting in decreased aggression among prisoners and in addition, developing aggression prevention interventions for children at risk. Although the relationship between childhood trauma and aggression had already been investigated in prisoners, the effect of cognitive control was not taken into account in previous studies. Childhood trauma, response inhibition and aggression were cross-sectionally examined in 157 Dutch male prisoners using retrospective questionnaires and performing an inhibition task (n=157 males; Mage =

38.17; SD= ± 12.7). They completed the trauma questionnaire ‘Jeugd Trauma Vragenlijst’ (JTV) and the self-report aggression questionnaire ‘Agressie Vragenlijst- Aangepaste versie’ (AVL-AV). In addition, response inhibition was measured by performing a Stop Signal Task. Although findings confirmed a significant relationship between childhood trauma and aggression (b= 0.198, p <.001), mediation analysis indicated that inhibition was not significantly related to childhood trauma (b= 0.100, p= .781), nor to aggression (b= 0.004, p = .698) and was therefore not a mediator in the relationship between childhood trauma and aggression (effect= 0.001; 95% BCa CI [-0.011 - 0.009]). Furthermore, emotional abuse was found to be the most associated with aggression (r= .367, p <.001), especially with hostility (r= .387, p <.001). In conclusion, the current study did not confirm a

mediating effect of inhibition in the relationship between childhood trauma and aggression. Further research is required to replicate these findings, as studies in prisoners are rare. The findings about the impact of emotional abuse may provide groundwork for future research.

Introduction

Aggression is an unacceptable behaviour that can be seen as a major public health problem (Sarchiapone, Carli, Cuomo, Marchetti, & Roy, 2009). It occurs in society, at work and even in prison and it expresses itself differently in men than in women (Anderson & Huesmann, 2003). Evolutionary theory and previous research have stated that men are much more likely to engage in physical

aggression than women (Daly & Wilson, 1988; Duntley, 2013). Moreover, problems with aggressive behaviour are especially prevalent among the forensic population (Sarchiapone et al., 2009).

Aggressive behaviour is generally considered to be complex and multidetermined (Coccaro, 2012). Discovering relationships to aggression is critical to understand and develop preventative measures of aggression. The objective of this study was to investigate two factors that may contribute to the development of aggressive behaviour, as some people seem to be more sensitive to develop aggression than others. The first factor investigated in this study was a history of childhood trauma (CT), since multiple studies have found a significant relationship between CT and aggression (Manly, Kim, Rogosch, & Cicchetti, 2001; Stouthamer-Loeber, Loeber, Homish, & Wei, 2001). To illustrate,

perpetrators of serious violence often have a history of being abused themselves (Van Erpecum, 2005). Furthermore, the second factor investigated was cognitive control, where having poor cognitive control appears to be associated with aggression (Caprara, Paciello, Gerbino, Cugini, 2007; Siever, 2008; Wilkowski, Robinson, & Troop-Gordon, 2010). The current study provides an investigation into individual differences in the development of aggression, with a specific focus on the relationship between CT and aggression, as well as considering the potential mediating influence of top-down cognitive control skills (Hawes et al., 2016).

Human aggressive behaviour, an observable manifestation of aggression can cause injury to self, others or objects (Coccaro, 2012; Sarchiapone et al., 2009). Aggression appears in several forms such as physical aggression, verbal aggression, anger and hostility (Hornsveld, Muris, Kraaimaat, & Meesters, 2009). In the current study, self-reported aggression was determined in male prisoners, taking these four categories into account. As mentioned before, aggression is considered to be multidetermined. Several causes underlying aggression are biological factors, social and physical environment (Sarchiapone et al., 2009). The development of aggressive and violent behaviour often starts in childhood (Van Erpecum, 2005). It has frequently been reported that trauma during childhood is associated with high levels of aggression (Manly et al., 2001; Stouthamer-Loeber et al., 2001).

Childhood trauma, often reported as childhood maltreatment or childhood abuse includes physical abuse, emotional abuse, sexual abuse and neglect, and is associated with a wide range of negative outcomes on psychosocial, physical, emotional and cognitive level (Pears, Kim, & Fisher, 2008). CT is held responsible for explaining 45% of the risk of developing psychopathology with an onset in childhood (Teicher & Samson, 2016). It is associated with higher prevalence of depression, psychosis, anxiety, substance abuse, eating disorders, altered cognitive functioning as well as with aggressive behaviour (Ball & Links, 2009; Norman et al., 2012; Teicher & Samson, 2013; Widom,

1989). Research carried out by Widom (1989) already indicated an association between abused or neglected children and delinquent, violent and adult criminal behaviour. In the years that followed, a lot of research has demonstrated the impact of CT on aggression. However, research investigating the effect of CT on later psychological and behavioural functioning mostly focused on physical and sexual abuse, whereas research on emotional abuse has lagged behind. The limited literature on emotional abuse found a predictive role for emotional abuse in the development of anger and irritability (Teicher, Samson, Anderson, & Ohashi, 2016; Zurbriggen, Gobin, & Freyd, 2010). Since it is well-established that emotional support in childhood is important in the development of later mental health and on the other hand, emotional abuse seems to be a potential precursor to mental health problems in adulthood, research on emotional abuse warrants special attention (Allen, 2011).

A body of retrospective and prospective research reported the deleterious effects of CT on functioning in adulthood and aimed to identify the underlying causes (Irigaray et al., 2013; Teicher et al., 2016). Recent evidence suggests that a history of CT can alter brain structure and function and is furthermore associated with altered neural activity (Insana, Banihashemi, Herringa, Kolko, & Germain, 2016; Teicher & Samson, 2016). To illustrate, the changes are adaptive responses of the brain in order to survive under adverse conditions (Teicher & Samson, 2016). Previous studies have shown that CT negatively affects frontal lobe regions that are involved in executive functioning and cognitive control, terms that are frequently used interchangeably in literature to describe the same concept (Insana et al., 2016).

Cognitive control is a collective term for mental processes that enables the nervous system to flexibly focus on actions that are currently relevant for goal-directed behaviour, while irrelevant actions are being inhibited (Badre, 2008; Kouneiher, Charron, & Koechlin, 2009; Miller, 2000). Working memory, cognitive flexibility and inhibition are the key components of cognitive control (Davidson, Amso, Cruess Anderson, & Diamond, 2006), which can be affected by a past of CT (Beers & De Bellis, 2002; Irigaray et al., 2013; Navalta, Polcari, Webster, Boghossian, & Teicher, 2006). These studies found evidence of CT-related impairment of executive functioning processes, for example planning, problem solving, working memory, information processing and response inhibition (Irigaray et al., 2013). In turn, a well-functioning cognitive control system is associated with lower levels of aggression and anger (Wilkowski et al., 2010). Nevertheless, a significant effect was often only found between aggression and inhibition, while the relationship between aggression and working memory and aggression and cognitive flexibility appears to be inconsistent (Ellis, Weiss, & Lochman, 2009; Kockler & Stanford, 2008; Raaijmakers et al., 2008). The current study aimed to investigate the role of cognitive control in the relationship between CT and aggression, in which only the key

component inhibition was investigated.

Despite a growing body of evidence of the relationship between CT and aggression, CT and cognitive control and cognitive control and aggression, the direct relation between these three

the relationship between a history of CT and aggression in Dutch male prisoners. Moreover, it was examined whether the relationship between CT and aggression in Dutch male prisoners can be mediated by inhibition. Since previous studies have shown that frontal lobe regions that are involved in cognitive control (e.g. inhibition) are affected by a history of CT and furthermore a well-functioning cognitive control system is associated with lower levels of aggression, it was expected that cognitive control acts as mediator in the relationship between CT and aggression. By incorporating CT and inhibition, perhaps one might get a clearer understanding in the development of aggressive behaviour in prisoners.

More insight in the relationship between CT, inhibition and aggression can contribute to the development of most effective intervention tools for decreasing the prevalence of aggression among prisoners. Effective interventions can lower the level of aggression and help decrease the risk of recidivism (Andrews et al., 1990; Antonowicz & Ross, 1994; Lipsey, 1992). This is an important goal since 47% of former prisoners reoffend within two years (Den Bak, Popma, Nauta-Jansen,

Nieuwbeerta & Jansen, 2018). Moreover, understanding the role of cognitive control in the relationship between CT and aggression could be helpful in developing effective methods for preventing development of aggressive behaviour in children and in turn possibly reduce the risk of developing violent criminal behaviour in later life. Furthermore, in order to develop targeted interventions, it was important to investigate what type of CT was most correlated with aggression. Based on this information, a distinction could be made between individuals with a history of CT. This way it is more clear which individuals benefit most from intervention. The CT-type that emerges as the one that is most correlated with aggression can be used in further analyzes that are looking at which type of aggression this CT-type is most related to. Moreover, both prevention and intervention tools could also be developed based on the different types of aggression. In sum, investigating what type of CT is most correlated to aggression and what type of aggression is most correlated with this type of CT, can be helpful in developing appropriate prevention and intervention tools to ultimately reduce aggression. This was investigated by performing additional analyses.

The purpose of this study was to explore the role of CT and inhibition on aggression. Another purpose was to investigate what type of CT is most correlated with aggression. This was explored by investigating the following hypotheses: Initially, it was hypothesized that a history of CT increases the level of aggression (hypothesis 1). Subsequently, it was investigated whether more CT may decrease the level of inhibition (hypothesis 2). Next, it was expected that a lower score on inhibition may increase the level of aggression (hypothesis 3). Moreover, inhibition was hypothesized to be a mediator in the relationship between trauma and aggression (hypothesis 4). In additional analyses, it was investigated what CT-subtype has the highest impact on aggression, in which emotional abuse was expected to be the most correlated with aggression (hypothesis 5). Lastly, it was investigated what type of aggression is most influenced by emotional abuse, in which it was hypothesized that anger is mostly correlated with emotional abuse (hypothesis 6).

Methods

2.1 Design

The current study was part of a larger project, which investigated the psychosocial criminogenic factors and neurobiological characteristics in Dutch male detainees (Den Bak et al., 2018). This project was conducted at the request of the ‘Wetenschappelijk Onderzoek en

Documentatie Centrum’ (WODC), which wanted to have more insight into the criminogenic factors in prison population. In order to answer the research questions of the current study, cognitive skills were tested cross-sectionally. Furthermore, retrospective self-reports were used to assess the level of childhood trauma and aggression.

2.2 Participants

Approximately 285 male prisoners were recruited (≥18 years old) in Penitentiaire Inrichting Nieuwegein, Justitieel Complex Zaanstad, PI Leeuwarden, PI Sittard, PI Grave and PI Lelystad. Multiple inclusion criteria were taken into account. Only prisoners that had been in prison for more than ten days were considered eligible for this project. Besides, only prisoners that were willing to participate and signed informed consent were included. In the current study, only participants that completed the Jeugd Trauma Vragenlijst (JTV) and Agressie Vragenlijst – Aangepaste Versie (AVL-AV) and performed the Stop Signal Task (SST) were included. This resulted in 157 participants (n=157 males; Mage = 38.17; SD= ± 12.7).

2.3 Measures

Aggression

To determine the level of aggression prisoners completed a 12-item version of the Agressie Vragenlijst (AVL-AV). The AVL-AV is a self-report questionnaire and is a Dutch translation of the Aggression Questionnaire Short Form of Buss and Perry (BPAQ-SF; Buss & Perry, 1992; Hornsveld et al., 2009). The 12 items comprise four subscales: physical aggression, verbal aggression, anger and hostility (Hornsveld et al., 2009). Both subscales and total scores were used in this study. The items were rated on a 5-point scale (1= completely disagree to 5= completely agree). Higher scores indicate higher levels of aggression. Several items provide information about contemporary situations, while others provides information about aggression during life time. The AVL-AV was found to show a good validity and good internal consistencies: for the total AVL-AV scale Cronbach’s α varied from .72 to .88 for three reference groups (Hornsveld et al., 2009).

Childhood trauma

To measure childhood trauma (CT) a modified Dutch version of the Childhood Trauma Questionnaire (CTQ) was used: Jeugd Trauma Vragenlijst (JTV) (Bernstein et al., 1994). The JTV is a 25-item-self-report retrospective assessment of traumatic experiences during childhood. Items were scored using a 5-point scale (1= never true to 5= very often true). The 25 items are divided into 5

categories: emotional abuse, physical abuse, sexual abuse, physical neglect and emotional neglect (Bernstein & Fink, 1998). JTV total scores as well as scores on each subscale were used in this study. Higher scores indicate more traumatic experiences. The JTV has a good reliability for both clinical and non-clinical populations. Researchers found a Cronbachs α of .91 for the subscale Physical abuse, .89 for emotional abuse, .95 for sexual abuse, .63 for physical neglect and .91 for emotional neglect. A study among 488 psychiatric patients found a good validity of the JTV (Thombs, Lewis, Bernstein, Medrano, & Hatch, 2007).

Cognitive control – Response inhibition

In order to measure response inhibition the Stop Signal Task (SST) was used. Frequently two ‘go’ stimuli (square and dot) were presented 250 ms in turn on the computer screen to set up a prepotent response tendency. However, in 25% of trials stimuli were followed by an auditory ‘stop’ signal that was presented irregularly to withhold their response. In stop-trials, the stop signal follows after a variable time (Stop Signal Delay: SSD). The SSD starts at 250 ms and is continuously adjusted by the respondents responses. When the inhibition was successful, the SSD reduced 50 ms and if not, increased with 50 ms (Verbruggen & Logan, 2008). The latency of the stop process, or an estimate of the time that it takes the participant to inhibit the initiated response, can be called Stop Signal Reaction Time (SSRT). A higher SSRT is related to a reduced response inhibition (Eagle et al., 2008).

Control variables – demographic variables

Multiple factors are found to be associated with the level of aggression, such as age and nationality. Aggressive behaviour seems to occur in different forms across life span (Liu, Lewis, & Evans, 2013). Besides, on the recommendation of Benish-Weisman (2016), cultural background was taken into account in the current study, since his study recognizes the role of nationality in supporting

aggression. With this in mind, nationality and age both were entered into the mediation analysis, as covariates.

2.4 Procedure

In 2017 preparations were undertaken to collect all materials, licenses and a request for ethical approval through the Ethics Committee (Leiden University) was submitted. Permission was granted by the Committee (Den Bak et al., 2018). In 2018, approximately 1300 male prisoners were asked to participate in the study and 283 actually participated. They took part in the study for approximately two hours. During the first stage of the study participants were presented with, and asked to sign an informed consent. Then the procedure of the study was explained. Participants were asked to fill out questionnaires and a neuropsychological test battery was administered. In the present study only answers to JTV and AVL-AV questionnaires were used in the analyses, as well as scores on the neuropsychological test Stop Signal Task.

2.5 Statistical analyses

Statistical analyses were conducted using ‘IBM Statistics 24’. Before conducting the analyses, cases with missing data were removed. Removing the participants with missing data did not have an effect on the results, since the MCAR-test showed that the data were missing at random (X2_{= 11.450,} p=.170). The data was then checked for outliers using Z-scores and boxplots. Assumptions of linearity

and normality of both dependent and independent variables were checked. Due to the low prevalence of CT, data on this variable was right-skewed. Due to the high number of participants (N=157), the central limit theorem (CLT) was used. CLT is a statistical theory that states that when the sample size gets larger (usually >30), the sampling distribution approximates a normal distribution, regardless of population distribution shape. Even when variables were not normally distributed in the population from which the sample was drawn (Field, 2013). In order to deal with the skewed data, bootstrapping was used. Since the bootstrapped distribution is normally distributed, the original data can be used without taking the underlying distribution into account. Additionally multicollinearity of the

independent variables was determined by assessing VIF values and homoscedasticity of residuals were checked.

Figure 1. Mediation model for the relation between CT and aggression, mediated by inhibition.

In the current study, the PROCESS macro (Hayes, 2012) was used to investigate the first four hypotheses. It was examined whether CT is related to aggression (hypothesis 1), whether CT is related to inhibition (hypothesis 2), whether inhibition is related to aggression (hypothesis 3) and lastly, whether inhibition acts as a mediator in the relationship between CT and aggression (hypothesis 4). A simple mediation regression analysis, model 4, was used with aggression as the dependent variable, CT as the independent variable and inhibition as the potential mediator. This model provides insights in the relationship between CT and aggression and the same relationship through inhibition. In this model, a total of 5000 bootstrap samples were calculated for 95% bias-corrected and accelerated (BCa) confidence intervals (Cis) and was then tested for model 4. The model is significant when the 95% CI excludes zero. A conceptual model is presented in figure 1. In this conceptual model the covariates age and nationality are not depicted.

Pearson’s correlations were determined using bivariate analyses in order to investigate hypothesis 5, in which it was expected that emotional abuse has the highest correlation with aggression, thus has the highest correlation with aggression. Besides, it was investigated how many variance in total aggression can be explained by emotional abuse and moreover by the five CT-subscales.

Hypothesis 6, in which it was expected that emotional abuse is most correlated with the subtype anger of aggression, was investigated using Pearson’s correlations which were obtained by bivariate analysis.

Results

An insight has been gained in the minimum scores, maximum scores, means and standard deviations among CT, aggression, inhibition and covariates to get a better perception of the data used in this study. The demographic data is presented in table 1.

Table 1.

Descriptive statistics of variables inhibition (SSRT), CT, Aggression and covariates (N=157)

Variable Minimum Maximum Mean SD

Total Childhood Trauma 25 109 41.25 17.25

Emotional Abuse 5 25 8.31 4.49 Physical Abuse 5 25 7.94 4.84 Sexual Abuse 5 24 6.34 3.73 Emotional Neglect 5 25 10.70 5.22 Physical Neglect 5 20 7.96 3.44 Total Aggression 12 57 27.69 10.37 Physical Aggression 4 19 9.81 4.12 Verbal Aggression 2 10 4.08 1.99 Hostility 3 15 6.92 3.29 Anger 3 15 6.89 3.32 Inhibition (SSRT) 62.0 537.7 287.02 72.76 Control Variables Age 18 73 38.17 12.65 Nationality - - - -

Model 4 of PROCESS was used in order to investigate the direct and indirect effects of the simple mediation. The results consist of the relationship between CT and aggression (c-path), the effect of CT on inhibition (a-path), the relationship between inhibition and aggression (b-path) and the

association between CT and aggression, through inhibition (c’-path). The results are displayed in figure 2, table 2, 3 and 4.

Figure 2. Direct and indirect effects of CT on aggression through inhibition.

Table 2 shows the results of the regression analysis with aggression as the dependent variable and CT as the independent variable (c-path). CT has a significant positive relationship with aggression among prisoners (b= 0.198, t(153) = 4.36, p <.001). The covariates age and nationality did not predict Y significantly.

Table 2.

Total effect model (N=157)

B

Aggression

SE t p

Constant 23.861 3.21 7.42 <.001

Childhood Trauma (c-path) 0.199 0.05 4.36 <.001

Covariates

Age -0.115 0.06 -1.85 .067

Nationality 0.009 0.37 0.02 .981

F(3, 153)= 7.35, p <.001, R2 _{= .13}

The regression analysis with inhibition as the dependent variable and CT as the independent variable (a-path) revealed that CT was not significantly related to inhibition (b= 0.100, t(153)= 0.28,

p= .781). The covariates age and nationality did not predict inhibition significantly. The results are

displayed in table 3. It was noticed that inhibition could no longer be a mediator in the relationship between CT and aggression, since the a-path was not found to be significant.

Table 3.

Results of mediation analysis: mediator variable model: inhibition.

B Inhibition- SE SSRT t p Constant 273.197 25,47 10.72 <.001 Childhood Trauma 0.100 0.36 0.28 .781 Covariates Age 0.156 0.49 0.32 .752 Nationality 0.744 2.92 0.26 .799 F(3, 153) = .08, p = .97, R2 _{= .002}

Moreover, the regression analysis with aggression as the dependent variable and inhibition as the independent variable (b-path) revealed that inhibition was not significantly related to aggression (b= 0.004, t(152)= 0.39, p = .698). The findings of the analysis are shown in table 4. Although inhibition could not be a mediator in the relationship between CT and aggression, the analysis has been further explained.

Table 4.

Results of mediation analysis: dependent variable aggression.

B

Aggression

SE t p

Constant 22.775 4.27 5.34 <.001

Childhood Trauma (c’-path) 0.198 0.05 4.32 <.001

Inhibition-SSRT 0.004 0.01 0.39 .698

Covariates

Age -0.115 0.06 -1.85 .066

Nationality 0.006 0.37 0.02 .987

F(4, 152)= 5.52, p <.001, R2 _{= .13}

Effects of CT on aggression B Boot SE 95%CI Direct effect 0.198 0.05 [0.108 - 0.288] Indirect effect 0.001 0.01 [-0.011 – 0.009]

When looking at the mediational effect of inhibition, the direct and indirect effect were determined. Table 4 and figure 2 show the direct and indirect effects between CT and aggression; CT and aggression, through inhibition. The direct effect (c’-path) between CT and aggression was b= 0.198, t(152)= 4.34 at p <.001; 95% BCa CI = [0.108 – 0.288]) and is significant. On the other hand,

the indirect effect of CT on aggression through inhibition is not significant (b= 0.001; 95% BCa CI [-0.011 - 0.009]), since the 95% confidence interval contains zero. In fact, hypothesis 4 was already rejected due to CT not having a significant effect on inhibition and inhibition not having a significant effect on aggression.

Additional analyses were performed in order to investigate what type of CT is most correlated with aggression and furthermore what type of aggression is most correlated with this type of CT (hypotheses 5 & 6). Furthermore, the variance in total aggression that can be explained by both emotional abuse and the five CT-subscales, was determined. Hypothesis 5, that emotional abuse has the highest correlation with aggression, was investigated using a correlation analysis. The results are displayed in table 5. Emotional abuse has the highest correlation with aggression and seems to be the most important CT subscale to explain the variance in total aggression (r= .367, p <.001). Moreover, it can be noticed that every variable in table 5 is significantly correlated with the dependent variable aggression at the 0.05 level, except for the covariates.

Table 5.

Pearson correlations of CT subscales and covariates on aggression

Variables Aggression p (1-tailed)

Aggression Emotional abuse - .367** <.001 Physical abuse .227** .004 Sexual abuse .178* .025 Emotional neglect .262** .001 Physical neglect .247** .002 Covariates Age -.131 .103 Nationality .016 .841

* Correlation is significant at the 0.05 level (1-tailed) ** Correlation is significant at the 0.01 level (1-tailed)

Since a significant relationship was found between CT and aggression, multiple regression analyses were performed to investigate the proportion of variance in total aggression explained by emotional abuse and additionally by the 5 CT-subscales. The results are shown in table 6. The amount of unique variance in total aggression emotional abuse accounts for is 13.5% and is statistically significant (p <.001). Additionally, the variance in total aggression can be explained by the five CT- subscales for 14.6% (table 6). The overall regression model was significant, F(5,151)= 5.17, p <.001,

R2_{= .146. Since the covariates age and nationality were not found to predict aggression, they were not}

Table 6.

The variance in total aggression explained by multiple variables

Variables R2 _{% P}

EA, PA, SA, EN, PNa

Emotional abuse .146 .135 14.6 13.5 <.001 <.001

a_{. Emotional abuse (EA), Physical abuse (PA), Sexual abuse (SA), Emotional neglect (EN), Physical} neglect (PN)

b. _{Dependent variable: Total Aggression score}

Given the fact that emotional abuse was found to be the most correlated to aggression (table 5), the correlation between emotional abuse and the different types of aggression was investigated. It was expected that emotional abuse is most correlated with anger (hypothesis 6). The results of the bivariate analysis are displayed in table 6. The correlation between emotional abuse and anger was not found to be the highest, instead, emotional abuse and hostility were found to be the most correlated compared to other types of aggression (r= .387, p<.001). These findings are displayed in table 7.

Table 7.

Pearson correlations of emotional abuse and covariates on different types of aggression

Outcome variables Emotional abuse p (1-tailed) Physical Aggression Verbal Aggression .366** .234** <.001 .003 Hostility .387** <.001 Anger .168* .036

* Correlation is significant at the 0.05 level (1-tailed) ** Correlation is significant at the 0.01 level (1-tailed)

Discussion

Human aggression is considered as a major public health problem and is especially prevalent in a forensic setting. A history of CT can be linked to the development of aggression, but the exact causes underlying this relationship have remained relatively unexplored (Sarchiapone et al., 2009). The current study aimed to investigate the role of inhibition in the relationship between CT and aggression. A better understanding of this role can contribute to the knowledge of how to decrease aggression and can ultimately help developing effective intervention tools for decreasing the prevalence of aggression among prisoners and preventing development of aggressive behaviour in children.

The main question of this study was whether inhibition acts as a mediator in the relationship between CT and aggression. However, no relationship was found between inhibition and CT and neither between inhibition and aggression. Inhibition was not found to be a mediator in this relationship. Several hypotheses were tested to find this result.

First, the hypothesis was tested that a history of CT is linked to higher levels of aggression. As predicted, CT was positively correlated with aggression and this effect seems robust and holds even after controlling for age and nationality. This finding is consistent with many previous studies (Allen, 2008, 2011; Sarchiapone et al., 2009; Zurbriggen et al., 2010).

In addition, this study examined the relationship between CT and inhibition (hypothesis 2). It was expected that a higher level of CT was correlated with a lower score on inhibition, but current findings did not confirm this relationship. A possible explanation could be that the effect of CT on inhibition seems to depend on, at least, two factors: the age of onset and chronicity of CT during childhood (Cowell, Cicchetti, Rogosch, & Toth, 2015). Cowell et al. (2015) showed that performance was significantly worse in children maltreated during infancy than in children maltreated later in childhood. CT during infancy seemed to have a more negative impact on the development of self-control, in which response inhibition is an important skill. A possible explanation for this effect was given by Johnson (2011). The first year of life is thought to be an important period of neuronal growth in which the brain’s sensitivity to external disruptions will increase. During these periods of rapid growth of neuronal connections, children can be especially vulnerable to the negative effects of trauma (Cowell et al., 2015). Furthermore, chronicity of CT was found to create a decline in inhibitory control (and working memory) performance. Children performed significantly worse when they were exposed to chronic trauma, whereas non-maltreated children and children who experienced CT during a single period of development performed equally well. These factors were not taken into account in this study due to the fact that the JTV-questionnaire does not provide this information. Including these factors in future research will provide a better understanding in the relationship between CT and cognitive control (Cowell et al., 2015). Nevertheless, several studies did find a significant relationship between CT and inhibition by using this questionnaire, which undermines the aforementioned possible explanation (Marshall et al., 2016) .

Furthermore, the current study investigated the effect of inhibition on aggression. Hypothesis 3 predicted that a lower score on inhibition, would lead to higher levels of aggression. However, this could not be supported by the current findings, as no significant relationship between inhibition and aggression was found. Although many studies do not support the current results and claim the relationship between inhibition and aggression, a few studies support the current findings (Enticott, Ogloff, Bradshaw, & Daffern, 2007; Lee & Hoaken, 2007). These studies, however, do not offer an explanation for their findings.

A possible explanation for the lack of a significant relationship between inhibition and aggression could be the use of an inhibition task assessing ‘cold’ executive function (EF) inhibition.

‘Cold’ or ‘cool’ refers to a neutral, safe situation in which an individual is in a minimal emotional (neutral) state (De Brito, Viding, Kumari, Blackwood, & Hodgins, 2013). Higher and more complex brain functions can be used in this neutral state, resulting in reflective behaviour (Perry, Griffin, Davis, Perry, & Perry, 2018). On the other hand, ‘hot’ EF is used in a threatening, stressful and emotional situation. ‘Hot’ EF refers to self-management skills that are used in situations where emotions run high (De Brito et al., 2013). The more threatened an individual feels due to the situation, the more their functioning shifts from higher brain networks to lower, more basal networks. Consequently resulting in more primitive thinking and behaving, which is a more reactive way of behaving and this can manifest itself in a reduced inhibition response (Perry et al., 2018). Although no relationship was found between ‘cold’ EF inhibition and aggression, there could possibly be a relationship between ‘hot’ EF inhibition and aggression. From this it can be concluded that the ability to inhibit behaviour depends on the emotional state of an individual and therefore it depends on the situation whether someone is able to inhibit his behaviour. Moreover, Perry et al. (2008) found that individuals with a history of CT are more sensitive to threatening and stressful situations, because they interpret stimuli as threatening more quickly. Individuals with a history of CT thus use the more primitive brain functions more often, resulting in impulsive, often aggressive reactions (Perry et al., 2018).

Furthermore, aggression often occurs in situations in which individuals feel threatened and thus make use of ‘hot’ EF. Future research should therefore investigate the relationship between ‘hot’ EF inhibition and aggression.

Investigating the aforementioned 3 hypotheses was the base for investigating the main question and hypothesis (4), in which inhibition was expected to be a mediator in the relationship between CT and aggression. Since inhibition was not found to be significantly related to CT nor to aggression, inhibition could not be a mediator in their relationship. Although previous studies suggest relationships between CT and aggression, CT and inhibition, and inhibition and aggression, the current study is the first to directly investigate the mediational effect of inhibition in the relationship between CT and aggression. Only one literature review described that this relationship appears likely, but mentioned that no reports have yet been proposed explaining the underlying mechanisms. Due to the lack of studies investigating this relationship, more research is required. This study was unable to encompass all factors possibly effecting the relationship, such as substance abuse, age of CT onset and CT chronicity (Evren et al., 2015), so additional analyses are recommended incorporating more control variables. However, investigating the relationship between aforementioned ‘hot’ EF inhibition and aggression seems more important in future research.

Furthermore, this study investigated what CT- subtype has the highest correlation with aggression. The current finding, that emotional abuse has the highest correlation with aggression, was consistent with previous studies (Allen, 2011; Sansone, Leung, & Wiederman, 2012; Zurbriggen et al., 2010). This study highlights the importance of emotional abuse as a risk factor for later aggression. Although this effect was found, more studies are necessary to replicate this finding. An overwhelming

majority of previous studies on the effect of CT on aggression have focused on sexual abuse or physical abuse/neglect, but the effect of emotional abuse was rarely studied. The few studies on emotional abuse did not show a relationship with aggression. Instead, these studies consistently found emotional abuse to be a predictor of anger and irritability (Teicher, Samson, Polcari, & McGreenery, 2006; Zurbriggen et al., 2010). A limitation of these studies was the cross-sectional nature of the design to measure CT. Only few longitudinal studies exist that offer suggestions for next steps in research on emotional abuse (Egeland, Yates, Appleyard, & Dulmen, 2002) . For instance,

relationships between different types of CT and aggression were found, but emotional abuse was not included. Given the results of the current study, as well as previous studies demonstrating the effect of emotional abuse (Allen, 2008; Miller-Perrin, Perrin, & Kocur, 2009; Spertus, Yehuda, Wong,

Halligan, & Seremetis, 2003), more research on childhood emotional abuse appears warranted in future longitudinal studies (Allen, 2011).

In hypothesis 6 it was expected that emotional abuse is most correlated with subtype anger of aggression. However, the highest correlation of emotional abuse was not with anger, but with hostility. The correlation with anger was significant, but less than hostility. This outcome was not in line with hypothesis 6. Nonetheless, since a significant correlation was found between emotional abuse and anger, the findings did not entirely contradict previous studies. Research carried out by Eckhardt, Barbour & Stuart (1997) indicated that anger and hostility are difficult to differentiate and these terms are often used interchangeably. The aggression questionnaire used in the current study is a shortened version of the Aggression Questionnaire (Buss & Perry, 1992), in which four subscales of aggression were measured. Previous confirmatory factor analysis indicated that when this questionnaire is used in an offender population, the four-factor aggression model did not fit the data. This means that the four factors are difficult to distinguish in an offender population (Williams, Boyd, Cascardi, & Poythress, 1996). Since a shortened version of the Aggression Questionnaire was used, with fewer items, it is conceivable that it is even more difficult to distinguish these four factors.

A limitation of the current study was the use of self-report questionnaires for both independent and dependent variables. Aggression is hard to determine with self-report questionnaires. First of all, prisoners could minimize their own problems with impulse-control. Secondly, high rates of psychiatric disorders occur among prisoners, often leading to a limited self-insight and social desirability

(Recklitis & Noam, 2004). Since the self-assessment of aggressive behaviour largely depends on these two characteristics, it can cause problems for the validity of the self-assessment of aggressive

behaviour (Nijman, Bjørkly, Palmstierna & Almvik, 2006; Recklitis & Noam, 2004). However, the validity and reliability of both AVL-AV and JTV questionnaires were considered sufficient.

Nonetheless, self-report questionnaires always remain sensitive to biases and social desirability. Prior to the study and also during the study, prisoners were reminded of the anonymity of the test results. Nevertheless, some prisoners remained suspicious, possibly resulting in socially desirable answering.

Another possible limitation is the use of multiple choice questionnaires. Several authors have argued that ‘production tools’ should be used instead of ‘recognition tools’ like multiple choice

questionnaires in forensic settings (Gavaghan, Arnold, & Gibbs, 1983; Stams, Brugman, Dekovic, Van Rosmalen, Van der Laan, & Gibbs, 2006; Van Vugt et al., 2011). Production tools include

questionnaires in which respondents were asked to write down their responses to statements or

pictures. In contrast to multiple choice questionnaires, these free-answering questionnaires are thought to provide more direct information about the respondent’s behaviour, which is not influenced by thinking about their own score on the Likert scale. Although it seems useful to work with free-answering questionnaires in future studies, it must be considered whether their application is also feasible since using them in practice is time-consuming (Drenth & Sijtsma, 2006).

Moreover, another potential limitation of this study is the fact that prisoners with serious psychiatric problems were not included, because they often stay in penitentiary psychiatric centres. This means that prisoners with severe behavioural problems may be less represented in this study, which could possibly distort the result of the analysis. Furthermore, the reader should bear in mind that other components of cognitive control (such as working memory) were not examined in the current study. For this reason overall conclusions regarding cognitive control cannot be made.

Nevertheless, this study also has several strengths. One of the strengths was the sufficiently large sample size in which the research was conducted, ensuring adequate statistical power. Another strength of this study was the own willingness of prisoners to participate in this study. Prisoners were not obliged to participate, but were rewarded if they did. It is conceivable that voluntary participation in research provides more reliable answers. Additionally, the data was collected and scored by students, after which the scores and interpretation were checked by other students. As a result of this extra check, it can be assumed that the answers on the questionnaires and the test results have been correctly copied, which will benefit the reliability of the current study. Nevertheless, this study does not provide enough knowledge to implement into practice, but it does show the importance of the effect of (emotional) CT on the development of aggressive behaviour.

In conclusion, knowledge about the role of cognitive control in the relationship between CT and aggression is not yet sufficient, nor is the role of emotional abuse in the development of

aggression. Nevertheless, the findings and limitations of this study offer a number of ideas for future research. Although the current study did not find a mediating effect of inhibition in the relationship between CT and aggression and the statistical power of this study is sufficient, replicating studies are necessary in order to exclude the effect of inhibition. Furthermore, more insight is needed into the effect of emotional abuse on aggression. As mentioned before, CT and aggression are multifaceted experiences that include (predisposing) factors as age of onset and chronicity of trauma. To improve future research, it may be beneficial to take these factors into account in future studies. In addition, a longitudinal study design can be beneficial regarding reliability of the results.

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