One of the key findings of this study was the prominent disparity in the Neuroticism dimension, with BD II patients scoring significantly higher than BD I patients. In addition, scores on the lower-order facets of the Neuroticism dimension, to wit Anxiety, Depression, Self-consciousness, and Vulnerability, differed significantly between the two groups. Akiskal et al.33
, in accordance with our finding, have reported that BD II patients had high Neuroticism scores, whereas BD I had low scores on this dimension, although the cited study measured Neuroticism in a manner distinct from the tests used in the present study. Neuroticism is the predisposition to experience psychological stress, as manifest by depression, anxiety, or other negative affect.34
Our results therefore suggest that patients with BD II have greater psychological vulnerability than do patients with BD I, so that stress more easily leads to maladaptive reactions and depression in BD II patients. Neuroticism is well known to be associated with certain clinical features of depression, including greater chronicity and severity, and a higher incidence of recurrence.35-37
As in unipolar depression, it is plausible that higher Neuroticism may have a negative impact on the outcome of BD II; however, this speculation need further evidence to be proved.
We found that BD II patients scored lower than did BD I patients on Extraversion, which is consistent with previous results.33
The lower-order facet of Extraversion, Positive emotion, was also lower in BD II than in BD I patients. Extraversion is associated with the quantity and intensity of energy directed outwards into the social world and encompasses traits such as sociability, activity, and the tendency to experience positive emotions.34
High scores on the Positive emotion facet are related to the experience of joy, happiness, love, excitement, and optimism.34
Based on the FFM description, our results indicate that patients with BD II may be less interested in social activities and have a lower capacity to experience positive affect than patients with BD I. Furthermore, BD II patients are less sociable and more likely to have negative affectivity than BD I based on our finding of the lower Extraversion score of BD II patients.
Achievement-striving striving and Competence facets were also significantly lower for BD II than for BD I patients. Achievement-striving captures the personality trait of need for personal achievement and sense of direction.34
This facet could be conceptualized as an aspect of positive affect regulation.38
A prospective study in BD I patients showed that a high Achievement-striving facet score predicted increases in manic symptoms.39
This facet is regarded as a propensity of BD I rather than BD II patients. Competence measures the tendency to believe in self efficacy.34
A lower sense of competence could make individuals likely to indulge in pessimistic thoughts; thus, the lower score of BD II in these facets may reflect cognitive distortion and/or relatively low self-esteem than BD I.
BD II has been shown to be a categorically different entity than BD I in genetic,15-17
aspects. Thus, BD II is not likely integrated with BD I at the level of personality traits as well. If the two subtypes of BD exist in a spectrum, they should be much more similar than different in the personality traits measured in our study. However, we found that BD II and BD I patients were strikingly dissimilar on Neuroticism and Extraversion. Although our study design could not fully account for the etiological distinction between BD I and BD II, the distinct personality profiles of these two subtypes may suggest fundamental difference in trait(s) between BD I and BD II patients.
When we compared results in BD II patients with normative data, we found profound deviations in all five dimensions. Differences with large effect sizes were seen for Conscientiousness and Neuroticism, with medium effect sizes for Extraversion and Agreeableness, and a small effect size for Openness. BD I patients, however, showed less deviation from normative data, with small effect sizes for Neuroticism, Openness, Agreeableness, and Conscientiousness. This result is consistent with the notion that most BD I patients are sanguine and describe themselves as near-normal in Extroversion, whereas BD II patients exhibit greater mood instability potentially linked with temperamental dysregulation.33
Our results should, however, be interpreted in the context of methodological limitations. Firstly, the cross-sectional design of the present study was unable to determine whether the personality traits measured were premorbid traits or post-affective personality changes. For example, we could not exclude the possibility that the high Neuroticism observed in BD II patients was a pathoplastic consequence of the bipolar illness. That is, recurrent affective episodes might cause negative social, occupational, or economic consequences; thus patients with BD may be more likely to show high Neuroticism as the number of episodes increases. Further work with longitudinal observation is required to address this issue. In addition to the effect of preceding affective episodes on personality, residual depressive and/or hypomanic symptoms might also have influenced the measurement of personality traits. In the present study, we excluded patients who were not in remission from discrete affective episode to control for the effect of residual affective symptoms. However, this did not mean that our study subjects were completely free from any level of subthreshold affective symptoms, although diagnostically they had remitted from major affective episodes.
In addition, the effect of comorbid diagnoses, personality problems, or other relevant clinical characteristics may have affected the measurement of personality traits. Although we excluded patients with active other primary Axis I disorders except BD, some patients may have had pre-existing psychiatric symptom(s) that were not clinically recognized or screened out during the structured interview. In addition, it is the limitation of our study design that we did not adapt the measure coexisting anxiety symptoms which could influence on the personality assessment, in particular on Neuroticism. With regard to Axis II disorder comorbidities, we did not adapt the structured interview to detect personality disorders; rather, patients with clinically-diagnosed Axis II disorders were excluded from the study. Since the vast majority of studies on Axis II comorbidities did not differentiate between BD I and BD II patients,40-42
it is less likely that co-existing Axis II disorders account for the differences in personality traits found in the two subtypes. Thirdly, our use of a limited sample size may have impeded our ability to detect substantial but subtle differences in personality traits between the two BD subtypes. Lastly, our study results may not be generally applicable, in that our subjects were recruited from one specific tertiary hospital and thus may not be representative of BD patients in general.
In summary, our results clearly suggest that BD I and BD II patients have distinct personality which supports the separation in enduring trait dimensions between the two subtypes. The most evident differences were on measures of Neuroticism and Extraversion between BD I and BD II patients from the FFM perspective. Further studies, including longitudinal assessments, are needed to determine how these personality traits are differentially linked with the etiology and clinical expressions of BD I and BD II respectively.