The purpose of this study was to evaluate behavioral measures of impulsivity and aggression as potential endophenotypes for borderline personality disorder (BPD). Individuals with BPD did not differ from either the Axis II control group (OPD) or the healthy volunteer group (HV) on behavioral measures of cognitive impulsivity (PAT and BGT). The BPD group did show more motor impulsivity (IMT) and affective aggression (PSAP) in comparison to HV subjects (even after controlling for Axis I comorbidity within BPD), but not in comparison to the OPD group. In contrast, self-report measures of both aggression and impulsivity did, for the most part, discriminate BPD from both comparison groups. Our results suggest that the behavioral measures of impulsivity/aggression were not uniquely associated with BPD, but in the case of the PSAP and IMT, were associated with the illness, thus providing partial support for these measures as a potential endophenotype (Gottesman and Gould, 2003
). There was more support for the PSAP as an endophenotype for aggression within BPD samples, as the PSAP was related to self-reported aggression in the BPD group.
BPD subjects made a greater proportion of aggressive responses on the PSAP than HV subjects, even after controlling for Axis I psychopathology. This extends previous research (Dougherty et al., 1999
) and provides partial support for the PSAP as an endophenotype for BPD. However, this is limited by the finding that BPD subjects did not differ from their OPD counterparts in their level of PSAP aggression. The first and most obvious reason for this is that behavioral aggression is not sufficiently unique to BPD. Irritability and aggressiveness is a symptom of antisocial personality disorder (APA, 1994
), and several other personality disorders (e.g., narcissistic, paranoid and obsessive–compulsive personality disorder) are also associated with an increased risk of aggressive behavior (Berman et al., 1998
; Goldberg et al., 2007
; Kim et al., 2008
; Villemarette-Pittman et al., 2004
). However, most of this research relied on self-report aggression measures. With the exception of antisocial personality disorder (Moeller et al., 1997
), it is not clear which personality disorders other than BPD are associated with increased aggression on a behavioral task.
It is also possible that the aggression task used did not provide the correct context to distinguish individuals with BPD from those with other disorders. Most of the aggressive behavior exhibited by individuals with BPD occurs within the context of relationships with friends, associates and loved ones – relationships where a perceived insult or attack would be particularly threatening. Currently, all laboratory aggression tasks are based on an interaction with a stranger. If the PSAP cover task was modified to simulate aggression from an existing relationship, then groups differences between BPD and other PD’s may emerge. Provocation intensity and/or frequency may also have obscured potential differences between BPD and OPD groups. The current aggression task used a low intensity aggressive behavior (taking a small amount of money from an opponent). Lower intensity aggressive acts such as arguing or breaking something inconsequential are more ubiquitous and under less genetic control than more severe acts such as physical assault (Coccaro et al., 1997a
). It is possible that behavioral measures of physical aggression may better differentiate individuals with BPD. Conversely, the frequency of provocation used in the study was very high (the highest that is used on the PSAP). This was done to maximize group differences in responding over a single PSAP session. However, it is possible that this high frequency of provocation masked differences between OPD and BPD groups that may have existed at more moderate provocation frequencies.
Aggressive responding on the PSAP was associated with several self-report anger and aggression scales, supporting the utility of the PSAP as an endophenotype for aggression within BPD samples. In contrast, OPD and HV subjects showed no significant association between PSAP aggression responding and self-report aggression measures (there was a single trend level association between PSAP and LHA-Aggression for HV subjects), putting in question whether the PSAP is a valid measure of aggression for these groups. For HV subjects, a possible explanation is that there was insufficient variance in PSAP B button pressing to demonstrate a correlation with other measures. Supporting this, the HV groups only chose a B response on 3% of the trials and had approximately half the standard deviation of the other groups. The OPD group did show high rates of B button presses, but no relationship with self-reported aggression. The reasons for this are not entirely clear. OPD individuals in general may have poor self-knowledge about their aggression. Alternately, the heterogeneity of the OPD group may have obscured significant relationships between PSAP aggressive responses and self-report measure of anger/aggression for specific personality disorders. Remember, we found a high level of variability for correlations between PSAP and BPAQ among the different PDs in the OPD group. For at least a portion of the OPD group (e.g., those with schizophrenia spectrum personality disorders) PSAP B button presses may have been tapping into another symptom, such as paranoia or other cognitive problems.
BPD subjects demonstrated greater motor impulsivity (IMT) than the HV controls but not the OPD comparison group, providing partial support for the motor impulsivity task as an endophenotype for BPD. In contrast, BPD subjects were not significantly different from either HV or OPD subjects on behavioral measures of cognitive impulsivity (PAT, BGT). This was surprising in light of clinical evidence that individuals with BPD engage in impulsive decision making as well as previous research showing individuals with BPD are more impulsive on behavioral measures of cognitive impulsivity (Dougherty et al., 1999
; Hochhausen et al., 2002
). More prototypical behavioral measures of cognitive impulsivity such as Delay Discounting might have shown separation between BPD and comparison groups. However, it is also possible that increased cognitive impulsivity among subjects with BPD may be contextually dependent. Much of the behavior in patients with BPD identified as “impulsive” such as suicide attempts, self-harm and other self-destructive behavior occurs in response to acute negative affect (Brown et al., 2002
). In the absence of this negative affect, individuals with BPD may not be significantly more cognitively impulsive than other individuals.
There was modest evidence for behavioral impulsivity measures as an endophenotype for impulsivity within BPD. The IMT was correlated at a trend level with both trait motor and trait nonplanning impulsivity. The PAT was also correlated at a trend level with trait nonplanning impulsivity, while the BGT was significantly correlated with trait motor impulsivity. For OPD and HV groups, the relationship between behavioral and self-reported impulsivity was weaker. There were no significant correlations in the HV group and only one (BGT and LHIB clinically significant impulsivity) in the OPD group. Furthermore, in several cases the relationship between behavioral and trait impulsivity trended towards being higher for the BPD subject as compared to HV subjects. This is consistent with research showing modest, inconsistent relationship between behavioral and self-report impulsivity measures in non-clinical samples (Mitchell, 1999
; Reynolds et al., 2006
; Richards et al., 1999
In contrast to the laboratory measures, self-report measures consistently showed BPD subjects to be more impaired than either HV or OPD groups. Among the aggression measures, BPD subjects reported more trait physical aggression, verbal aggression, anger, and hostility on the BPAQ than OPD subjects who in turn reported more than HV subjects. An identical pattern was shown for the LHA aggression scale and four of the six STAXI scales (i.e., trait anger, anger in, anger out and anger expression) These findings support earlier research showing that individuals with a personality disorder tend to be angrier, more irritable and more prone to engage in aggressive behavior than those without any such psychopathology (McCloskey et al., 2006
), and that may be particularly true among individuals with BPD (Goodman and New, 2000
; Joyce et al., 2003
Individuals with BPD also reported higher levels of motor, attention and nonplanning impulsivity on the BIS than either OPD or HV subjects. Increased impulsivity was distinctly associated with pathologic activity as BPD subjects reported higher levels of clinically significant impulsivity and antisocial behavior than OPD or HV groups, though the three groups did not differ with respect to non-clinically significant impulsivity. This is consistent with earlier research showing individuals with BPD self-reported higher levels of trait impulsivity (Dougherty et al., 1999
; Paris et al., 2004
; Wilson et al., 2007
), as well as clinical observation that individuals with BPD have difficulty delaying gratification, make decisions quickly, and either devalue or fail to consider the consequences of their actions.
BPD women also reported more self-aggression than OPD and HV women, though BPD and OPD men did not differ from each other. We did not expect to find a gender by group interaction for self-aggression. Overall, non-lethal self-aggression (i.e., suicide attempts and self-mutilation) is equally to more prevalent in women (Briere and Gil, 1998
; Zlotnick et al., 1999
). Among patients with BPD, a history of self-aggression is also equally likely in either gender (Grilo et al., 2004
; Johnson et al., 2003
). One possible explanation for our finding is that, in contrast to the aforementioned studies that assessed only the presence or absence of the self-aggression BPD criteria, the LHA self-aggression scale assessed the number of self-aggressive acts. Thus, history of self-aggression may discriminate BPD from OPD across gender, but frequency of self-aggression does so only for women. To test this we performed an exploratory analysis in we dichotomized the LHA self-aggression data (no self-aggression vs. any self-aggression). We found that presence of any self-aggression discriminated BPD from OPD across both genders in our sample (p
Self-report measures were superior to behavioral task at discriminating BPD from comparisons groups. Despite this, self-report and behavioral measures of aggression/impulsivity were often correlated among BPD subjects, at least at a trend level. It is possible that deficits in emotional awareness (Levine et al., 1997
) as well as a dramatic “all or nothing” style of presentation (Rosenthal et al., 2007
), led BPD subjects to overestimate their impulsive/aggressive tendencies. Alternately, the pattern of results may suggest aggression and impulsivity is secondary to affective lability (a.k.a. emotional dysregulation) in BPD. Despite the heterogeneity of BPD symptoms, affective lability is believed to be a core dimension of BPD (Lieb et al., 2004
; Rosenthal et al., 2007
), and is the most prevalent and enduring of the BPD diagnostic criteria (McGlashan et al., 2005
). Aggression is typically a response to dysregulated anger and many of the impulsive and aggressive behaviors engaged in by individuals with BPD are associated with emotional distress (APA, 2000
). Furthermore, recent studies suggest that emotional state may moderate behavioral impulsivity in BPD (Chapman et al., 2008
). Therefore, though individuals with BPD have more trait aggression and impulsivity than comparison groups, actual aggressive or impulsive behavior would primarily occur only when they were experiencing significant negative affect. We know BPD and OPD groups did not differ in their reported level of state anger while completing the STAXI. It is unclear if the behavioral tasks used in this study were able to produce a significant increase in negative affect, as this unfortunately was not assessed. The unique signature behaviors of BPD may emerge from the interaction of emotional dysregulation and interpersonal sensitivity (e.g., as in context of termination of important relationship and ensuing feelings of abandonment) driving impulsive and aggressive behaviors. In this sense, the identification of endophenotypes for dimensions of emotional dysregulation and impulsivity/aggression (discussed here) in BPD would serve to validate the diagnosis but as the focus of this paper is specifically impulsivity and aggression in BPD, we will address these broader issues in a subsequent paper.
The results of this study provide limited evidence for the use of behavioral measures of aggression (PSAP) and motor impulsivity (IMT) as endophenotypes for BPD, and support the use of the PSAP as potential endophenotype of aggression within BPD samples. Strengths of the study include the use of a relatively large medication-free clinical sample, inclusion of a non cluster B personality disorder control group in addition to healthy volunteer group, use of multiple self-report and behavioral measures of impulsivity and aggression, and controlling for differences in Axis I psychopathology across diagnostic groups. However, aspects of the study may limit the generalizability of these results. There was a significant amount of missing behavioral impulsivity data due to late introduction of measures and software problems. This reduced our power to detect potentially significant group effects, though the BPD vs. OPD comparison did not approach significance for any of the behavioral impulsivity measures. Also, the OPD and BPD groups were heterogeneous with regard to Axis II personality disorders, including significant cluster B co-morbidity in the BPD group. However, the additional cluster B co-morbidity in the BPD group would be expected to, if anything, accentuate differences between OPD and BPD groups on behavioral aggression and impulsivity measures.
The behavioral measures of aggression and impulsivity we chose were selected on the basis of extensive past research implicating them in aggression and in some cases to BPD. However, it is possible that there are other measures that would better distinguish BPD from other clinical groups, perhaps by recreating the interpersonal and other contextual factors that would increase the negative affect associated with the task. Behavioral measures of other core components of BPD such as affective lability or interpersonal sensitivity (Lejuez et al., 2003
) may also serve as useful endophenotypes of BPD. It is also possible that the heterogeneity and complexity in BPD precludes any one behavioral measure from being a specific endophenotype of the disorder. In this case the need to identify genetically homogenous dimensions of BPD will aid in our understanding and treatment of the disorder, allowing for more targeted interventions, both pharmacological and psychosocial. By identifying endophenotypes for the major dimensions of BPD and their underlying genotypes we may be able to identify genetic/endophenotypic signatures for this disorder. Our study suggests the PSAP may be an effective endophenotype for the central trait or dimension of aggression in BPD. Accordingly, future gene mapping and family studies of BPD would be well served to include such a measure.