This study indicated that there was a statistically significant difference in regard to the traits of Neuroticism, Extraversion, and Conscientiousness between the individuals with MDD and outpatients without psychiatric disorders recruited from GP settings. The individuals with MDD had a higher score of Neuroticism, but a lower score of Extraversion, and Conscientiousness. Additionally, individuals with MDD who had depressive residual symptoms scored higher on Neuroticism and lower on Conscientiousness than the individuals with MDD who had not.
In regards to this study’s result, we found differences between personality traits among Thai individuals with MDD and outpatients without psychiatric disorders recruited from GP settings, especially in connection to the traits of Neuroticism, Extraversion, and Conscientiousness. The result was similar to those results reported by a prior meta-analysis, including 175 studies published from 1980 to 2007 [14] as well as to that of another study in the Netherlands [16], and the USA [17]; which also suggested that the presence of MDD was significantly associated with higher Neuroticism, and lower Extraversion and Conscientiousness [16, 17]. Trying to identify the association between personality traits and the age-onset of MDD, our study found that a reduction of the Neuroticism score and increments of the Openness score was associated with increments of the age-onset of MDD. These findings were opposite to the aforementioned study from the Netherlands, which found that the earlier onset of MDD was significantly associated with a higher Openness score [16]. A possible explanation for this discrepancy could be that there were age range differences, in our study their median age was 45 years while in the previous study, the participants were older adults.
An investigation study in regards to any causal relationship between the trait of Neuroticism and MDD via Mendelian Randomization (MR)identified strong evidence that the trait of Neuroticism was a causal risk factor for MDD, and every 1-point reduction in the Neuroticism score reduced the log odds of MDD by 0.25 [26]. Furthermore, a reduction of the Neuroticism score by 4 points reduced the chance of MDD by about 25.0% [26]. However, due to our cross-sectional study design, our result cannot support a causal relationship. Asides from looking at personality traits as risk factors for MDD, depression itself may have had a state effect due to its characteristic patterns of thinking, feeling, and behaving in a concrete situation at a specific moment in time, on personality scoring. Many prior studies found that individuals with MDD reported a higher score of Neuroticism when they were depressed than when they were not. Even though the scores changed after remission, the personality traits and characteristics tended to be well preserved [27, 28]. However, the state effect on personality assessment was worth noting.
According to depressive residual symptoms, our study identified that individuals with MDD who reported depressive residual symptoms had a higher score of Neuroticism and lower score of Conscientiousness than individuals with MDD who had no presence of depressive residual symptoms. This result is also consistent with a prior study in connection to a longitudinal analysis, on the improvement of older adults with MDD who received treatment for 3 and 12 months, which found a positive correlation between improvement and lower Neuroticism scores [17]. In another study, higher scores on the Openness domain at the beginning of treatment were associated with lower MDD severity at treatment completion [29]. Our study did not find any significant association between depressive residual symptoms and Openness scores, however, due to the cross-sectional study design, we cannot assess this point of information directly. Previously, only a few studies assessed the association between personality traits and depressive residual symptoms. One study found that remitted patients with residual nightmares scored higher on Neuroticism [30]. However, our result added some new knowledge to this area, as we found that treated individuals with MDD who had higher Neuroticism scores reported more frequent depressive residual symptoms such as the loss of interest, feeling down, sleep problems, feeling bad about self or low self-esteem, trouble concentration, and psychomotor retardation. Furthermore, treated individuals with MDD who had lower Conscientiousness scores reported more frequent depressive residual symptoms of loss of interest, feeling bad about self or low self-esteem, and trouble concentrating.
Finally, in the clinical field, knowing the personality traits of an individual with MDD potentially gave useful information to clinicians. The trait of Neuroticism was highly related to escape-avoidant coping interactions with life stressors that were related to developing MDD symptoms [18, 19]. Low Conscientiousness was related to a lack of problem-solving [20], which was found in MDD with the presence of depressive residual symptoms, as per our study. This was due to problematic coping, still interacting with everyday life stressors, and causing distress. Therefore, considering that personality traits might play a role in determining the choices of treatment for MDD. Individuals with MDD who had higher scores on Neuroticism were more likely to achieve remission by being treated with pharmacotherapy versus cognitive-behavioral therapy [29]. Individuals with MDD who reported depressive residual symptoms should be assessed in connection to their personality traits to aid in determining whether to provide additional pharmacotherapy or psychotherapy, and to more effectively target improper coping styles. Furthermore, knowing the personality traits of non-depressive individuals might help identify people who were potentially more prone to develop MDD in the future, and provide early preventative interventions.
This study had a few noteworthy strengths and limitations. To our knowledge, this was the first case–control study investigating the association of personality traits and MDD in Thailand. However, this study provided limited results and scope of interpretation due to its cross-sectional design, lack of a baseline measurement and long-term follow-up, as well as the utilization of self-administered questionnaires, with the possibility of misunderstandings regarding the intended meaning of the questions. Nevertheless, to minimize this, questionnaires with good reliability were utilized (good Cronbach’s alpha coefficient values). Other drawbacks were that our data were quantitative, and the sample size was limited to the patients enrolled at only one hospital. Hence, this dataset might not fairly represent Thai individuals with MDD countrywide. Additionally, there was a difference in occupation between groups, and the control group consisted of the outpatients without psychiatric disorders who were selected from a clinical setting. They were not part of the general population. This may have affected the relevance of this study’s results to the general population. Also, we were concerned about any transcultural effects on personality traits and MDD characteristics, but our observed results were largely congruent with prior studies abroad.
It is recommended that future studies include all individuals with MDD, from all regions of Thailand. In other words, a comprehensive multi-center study should be conducted. Moreover, future research should concentrate on different instruments, utilize more qualitative designs, and employ longitudinal surveillance or long-term follow-ups.
