Exposure to traumatic events and experiences: aetiological relationships with personality function

Abstract

Empirical research has shown that the odds of experiencing traumatic events are influenced by genetic factors and the heritability of trauma exposure varies with the type of trauma. Traumatic events per se are unlikely to be heritable; more likely to be inherited are factors such as personality that influence the person's risk for entering into, or creating, potentially hazardous situations. With data from 406 twin pairs (222 monozygotic and 184 dizygotic twin pairs) from the urban general population, the present study used multiple regression analysis to identify personality variables associated with exposure to trauma, and estimated the degree to which these relationships were mediated by genetic factors. The experience of violent assaultive traumatic events was predicted by antisocial personality traits, specifically juvenile antisocial behavior, self-harming behavior, Psychoticism (e.g. adult antisocial behavior and substance misuse), and being open to new ideas and experiences. Genetic factors were found to partially mediate these relationships as indexed by the genetic correlation coefficient. The values of the genetic correlations were statistically significant and ranged from 0.14 to 0.36, accounting for 5–11% of the observed correlation between personality and trauma exposure. These findings suggest that heritable personality characteristics explain part of the variance in the likelihood of exposure to some classes of traumatic events.

Introduction

Twin studies of posttraumatic stress disorder (PTSD) reveal that the symptoms of this disorder are moderately heritable. For identical and fraternal male twins who had experienced combat, genetic factors accounted for approximately 30% of the total variance in symptoms (True et al., 1993). Similarly, genetic factors accounted for 26–36% in the variance of PTSD symptoms in a predominantly female sample of twins from the general population (Stein et al., 2002).

The odds of experiencing traumatic events are also influenced by genetic factors (e.g. Lyons et al., 1993, Koenen et al., 2002, Stein et al., 2002), and the heritability of trauma exposure varies with the type of trauma (Stein et al., 2002). Factor analysis of a questionnaire assessing life events identified two types of traumatic events: (1) assaultive traumatic events, such as being mugged, sexually assaulted, or beaten up, and (2) non-assaultive trauma, such as severe motor vehicle accidents, the sudden unexpected death of a loved one, and the experience of seemingly random events such as natural disasters. Stein et al. (2002) found that exposure to assaultive traumatic events was moderately heritable (20% of the variance due to genetic factors), whereas the experience of non-assaultive trauma was not heritable. Thus, PTSD can be seen as a result of two sets of genetic and environmental susceptibility factors: (1) those influencing trauma exposure, and (2) those influencing the symptoms resulting from exposure.

Traumatic events per se are unlikely to be heritable. More likely to be inherited are factors that influence the person's risk for entering into, or creating, potentially hazardous situations. There has been a good deal of research on risk factors for experiencing PTSD symptoms. For example, preexisting personality traits such as neuroticism have been implicated (e.g. Fauerbach et al., 2000). Less is known about the personality traits predicting trauma exposure. Twin research by Kendler et al. (2002) has shown that neuroticism is associated with increased risk for exposure to some kinds of traumatic events. Personality traits other than neuroticism are also likely to play a role. For example, Kirkpatrick et al. (1998) reported that sensation seeking or novelty seeking was associated with increased risk for being a victim of rape. In the Vietnam Era Twin Registry, a sample that consists solely of males, preexisting conduct disorder (which might be considered an early manifestation of antisocial personality traits) was a risk factor for both trauma exposure and subsequent PTSD symptoms (Koenen et al., 2002).

The aims of the present study are two-fold. The first aim is to identify which personality variables are associated with exposure to trauma. The second, more fundamental, aim is to investigate whether the association between personality traits and trauma exposure has a biological basis. Put simply, are the genes for particular personality traits associated with a greater risk of experiencing particular types of trauma?