Strengths and limitations
For this paper, data were used from NEMESIS-2. Although for most parameters NEMESIS-2 is representative of the Dutch population, people with an insufficient mastery of Dutch, those with no permanent residential address and the institutionalized are underrepresented in this sample. Hence, our findings cannot be generalized to these groups.
BPD symptoms were measured with eight questions from the International Personality Disorder Examination (IPDE). Despite indications of the validity of these IPDE questions for assessing BPD diagnoses [
5], its properties make it unsuitable for use in clinical practice. However, the questionnaire can be used in epidemiological studies where the focus is on determining prevalence rates and associated correlates among groups of individuals. The present study yields clear indications of its validity, showing that respondents with a higher number of BPD symptoms are consistently more likely to report suicidal behaviour, all main groups of mental disorders, a vulnerability to these disorders, and a higher prevalence of mental disability. In order to establish the extent to which the present findings can be replicated, future population studies need to go beyond the exclusive use of screening questions and employ clinical interviews to assess full borderline personality disorder, as well as clinical reappraisal interviews to validate existing screeners for borderline personality disorder.
Just as in the NCS-R [
28], in our study one criterion for BPD (namely, recurrent suicidal behaviour, gestures or threats, or self-mutilating behaviour) was not assessed in the IPDE screening questionnaire. However, in the suicidality module of the CIDI 3.0, respondents were asked about whether or not they had ever had experienced suicidal ideation, or made suicide plans and/or attempts. As expected, the more BPD symptoms respondents had, the more often they reported having experienced suicidal behaviour in the preceding years; this confirms the validity of the IPDE questionnaire.
Underreporting and recall problems might conceivably have compromised the assessment of respondents of any BPD symptoms they might have and might have had, especially in those cases where such symptoms occurred a long time ago [
29]. The IPDE questions refer to rather stable personality characteristics (i.e. ‘What you are like most of the time?’ ‘What has been typical of you throughout your life?’). Mental disorders were assessed over a 3-year period, which could increase the likelihood of recall bias. Besides, it is conceivable that people with BPD symptoms perceive their mental health in a more negative way. We tried to minimize this type of bias by using a sound diagnostic instrument for the assessment of mental disorders. All in all, it is difficult to estimate the exact extent to which underreporting, recall or report problems may have influenced our findings.
In NEMESIS-2, the most common mental disorders were assessed. However, not all cluster B personality disorders were recorded in the dataset: the same goes for cluster A and cluster C personality disorders. This means that impairment associated with these disorders and the use of health care services could not be studied. Previous research has shown, however, that it is comorbid common mental disorders and not comorbid personality disorders that ultimately determine the social functioning of people with BPD in the general population; the same goes for seeking treatment [
10]. As this suggests, the associations found were hardly affected by our inability to adjust for other personality disorders.
Our study relates the number of BPD symptoms to comorbidity of common mental disorders and mental disability in the general population. Most population studies conducted thus far have mainly confined themselves to assessing the prevalence rate of BPD diagnosis and its associated correlates and consequences. Huang et al. (2009), for example, show that the prevalence rates of personality disorders (PD) differ between countries, although the sociodemographic correlates of PD and comorbidity with PD of different countries show great similarity [
18]. This implies that our findings on the prevalence rates of BPD symptoms can perhaps not be generalized to other countries. The associations between the number of BPD symptoms and sociodemographic characteristics, comorbidity of common mental disorders and/or mental disability, on the contrary, could be generalized to other countries.
Discussion of research findings
In our study, 1.1 % of the population studied reported ≥ 5 BPD symptoms. These individuals could be viewed as having BPD, because they fulfil the required number of DSM-IV criteria for a BPD diagnosis. This finding concurs with the assumption made by Lenzenweger et al. [
9] that the average population prevalence of BPD is circa 1 %.
The finding that a greater number of BPD symptoms is associated with less stable social situations (i.e. not living with a partner, or having no paid job) confirms earlier research (e.g. [
7,
10,
28]) and is partly inherent in the definition of BPD; after all, the main characteristic of BPD is that someone shows a pervasive pattern of instability in interpersonal relationships, self-image, and emotions.
One of the findings of our study was that the category of people with ≥ 5 symptoms consisted of a significantly higher proportion of females as compared to the categories of people with 0 or 1–2 symptoms. This contrasts with previous population studies, which have shown no gender differences in the prevalence rate of BPD (e.g. [
4,
5,
10]); yet it confirms the suggestion made in the DSM-5 that BPD is more common among women [
30] and also substantiates the findings of clinical studies, which have demonstrated that more women than men suffer from BPD. However, the gender difference in clinical studies may result from selection bias (i.e. women seeking health care more often than men do) [
31]. As a second point, additional analyses based on data from NEMESIS-2 have revealed that females and males differ significantly in terms of the type of BPD symptoms from which they suffer. Females more often report so-called ‘disturbed relatedness’ symptoms, in particular: ‘I often feel “empty” inside’ (7.9 % versus 3.6 %), and: ‘When I’m under stress, things around me don’t seem real’ (8.4 % versus 3.7 %); males, on the other hand, more often report: ‘I go to extremes to try to keep people from leaving me’ (10.4 % versus 7.0 %), which is a symptom of the affective dysregulation dimension [
32]. It is unclear whether the gender differences in the number of BPD symptoms found in the present study reflect any real differences, or whether women are more likely to report certain BPD symptoms.
Similar to previous findings (e.g. [
7,
8,
10,
18]), in our study the number of BPD symptoms turned out to be negatively related to age and education level. As regards age, this could result from the phenomenon of particular (i.e. impulsive) symptoms declining as people grow older [
2,
9]; the education effect could be a consequence of the disorder itself. After all, BPD is characterized by impulsive behaviour and an unstable pattern of interacting with others, which could impede educational achievements.
After adjustment for gender and age, respondents with a higher number of BPD symptoms were found to be significantly more likely to suffer from various mental disorders, including mood, anxiety and substance use disorders. This is consistent with previous research, which has shown that BPD is strongly comorbid with a variety of mental disorders (e.g. [
3,
5,
7,
8]) and is closely associated with both the distress sub-factor of the latent internalizing dimension and the latent externalizing dimension of common mental disorders [
33]. These findings lead us to question the extent to which common mental disorders and personality disorders should be viewed as distinct, and also the extent to which BPD can be distinguished clearly from normal variation [
12]. Future research might shed more light on these topics, which could have useful implications for both clinical practice and the mental health care structure as a whole. With the elimination of the multi-axial system in the DSM-5, some artificial distinctions between personality disorders and other common mental disorders (e.g. mood, anxiety and substance use disorders) have already disappeared.
The finding that, after adjustment for sociodemographic characteristics and comorbid mental disorders, higher numbers of BPD symptoms are uniquely related to higher mental disability scores is in line with previous research, which has found an independent contribution of BPD to mental disability [
7,
8] and an independent contribution of cluster B personality disorders to the impairment of role functioning and social interaction [
18]. However, our findings contradict another study, which has found largely reduced effects of BPD on impaired functioning after adjustment for comorbid mental disorders [
5].
As our study shows, people with a higher number of BPD symptoms are significantly more likely to use certain services to deal with their mental health problems; after adjustment for comorbid mental disorders, however, these differences largely disappear. This corroborates previous research, in which higher rates of use of mental health services were found for people with BPD as compared to people with any mental disorder [
11], and that people with any personality disorder often seek treatment for comorbid common mental disorders [
18]. However, it contradicts other research which shows that, after adjustment for demography and comorbid common mental and personality disorders, people with BPD are not more likely to seek help from mental health professionals [
5,
10].
All in all, our findings suggest that people with BPD seek help mainly for common mental disorders, even though most of their impairments do not result directly from these comorbid mental disorders. Thus, it seems that help-seeking as well as referral to specialized mental health care by general practitioners is often based on symptoms rather than traits, probably due to the perception, among both patients and professionals, of there being more effective treatment options available for common mental disorders than for BPD.