Resilience has been defined as an individual's ability to more effectively cope with stress and adapt in the face of adversity or trauma than other individuals.¹ Recently, the concept of resilience has been used worldwide and in various fields, from mental health to social sciences, because of unexpected terror attacks and frequent nature disasters.²³ Interestingly, resilience can be promoted through psychiatric intervention.⁴⁵

Over the past several years, training programs aimed at promoting resilience have been developed. A recent study on the US army suggested that such programs could improve resilience.⁶ Furthermore, recent studies found that a resilience program for the prevention and treatment of depression was effective in reducing depressive symptoms and protecting individuals against adversities.⁷⁸ And, a previous study showed that the resilience of non-clinical samples with mental disorders could be strengthened through intervention.⁹ Strengthening resilience is an important care strategy for patients with bipolar disorder because resilience is a protective factor against the recurrence of mood episodes among these patients.¹⁰ However, whether interventions that strengthen the resilience of people with bipolar disorder and unipolar depression differ is unknown.

In several countries, young healthy males are required to serve in the military, typically beginning in late adolescence to early adulthood. Conscripts could experience reduced stress levels, as a positive psychological reward, after successfully completing basic military training.¹¹ Furthermore, according to previous study of the US military and a recent meta-analysis study, physical fitness and religious activities could have a positive impact on resilience.¹²¹³ Therefore, the basic military training that Korean conscripts have experienced, such as regular physical fitness training reflecting their physical condition and religious activities, could cause improvements in resilience.

Previous studies have not sufficiently explained how resilience can be altered. Especially, it is not known whether the alternation of resilience is different among people with other mood disorders. Previous studies suggested the positive aspects of individuals with bipolar disorder, such as positive response in life stress, and lower risk for posttraumatic stress disorder (PTSD) in patients with history which had overcome the PTSD than in patients without a history of PTSD.¹⁴¹⁵¹⁶ Therefore, we hypothesized males with probable bipolar depression (PBD) would be more resilient than probable unipolar depression (PUD).

The aim of this study was to investigate whether the resilience of males with probable bipolar depression (PBD) can be strengthened and to compare it with that of males with probable unipolar depression throughout basic military training after controlling for depressive symptoms.

All participants were male. The mean age, education level and BIS-11-R scores did not significantly differ across groups (Table 1). A significant difference in the CDRISC scores throughout the course of basic military training was found between the three groups. The PBD group (65.7±13.8, 74.8±15.6) had significantly higher mean CDRISC scores than the PUD group at baseline and at week 5 (respectively, 65.7±13.8 vs. 58.2±13.6 and 74.8±15.6 vs. 60.8±15.5, p=0.000). The mean CES-D scores did not differ between PBD and PUD at baseline or the week 5 assessment. The PBD group was significantly higher in the change in the CDRISC mean scores than PUD group when the interaction of group and time was included in the model, and the effect size was moderate (p=0.009, F=6.967, η²_p=0.052). The group and time interaction did not result in a significant difference between the NC group and the other groups (Table 2, Figure 1).

This study investigated whether the resilience of males with PBD would be strengthened following the basic military training. To the best of our knowledge, the present study is the first to identify that basic military training could strengthen the resilience of males with PBD. The main findings of this study were that males with PBD had greater resilience than males with PUD at baseline, and that of males with PBD was more strengthened than that of males with PUD and that the former group experienced euthymia after military training. The effect size was moderate. Differences in resilience between those with PBD and PUD may be related to affective temperament or character traits.

Resilience was defined as positive adaptation to adversities and difficulties.⁹ Because conscripts undergo intensive training and live in a restricted environment during their service, conscripts experience psychological pressures during the early period of their military service.²⁵²⁶ In this context, strengthened resilience after completing military training in this study could represent a kind of psychological growth which means being increased self-confidence after adverse experiences.^11272829 An individual's resilience can be strengthened after he or she overcomes stress, especially if the individual has character traits such as self-transcendence and openness.²⁹³⁰³¹ Individuals with high levels of self-transcendence and openness become more resilient after a trauma because they are willing to accept failures and vent their emotions when they face adversities.²⁸³¹ Patients with bipolar disorder could also become confident after successfully completing a hard mission but not become excessively elevated.¹⁴ Because, patients with bipolar disorder might have more character traits that facilitate the strengthening of resilience, such as openness and self-transcendence, compared with patients with unipolar depression who have negative rumination.³²³³³⁴

Individuals with an affective temperament or character traits such as hyperthymic temperament, extroversion and high sociability have more characteristics that protect against stress.³⁵³⁶³⁷ In previous studies, patients with BD and vulnerable groups such as relatives of patients with BD have higher hyperthymic temperaments, extroversion, and sociality compared to the unipolar depression and NC group.³⁸³⁹ In evaluations of resilience, considering protective factors for trauma/adversity is important. The resilience of the PBD group was higher before and after the training than that of the PUD group because of the protective role of the characteristics of bipolar disorder mentioned above. In the present study, the more strengthened resilience found among males with PBD was likely affected by personality traits, as people with bipolar disorder are more resilient than those with unipolar depression.

Differences in neurocognition might inform differences in the strengthening of the resilience of people with PBD and PUD. Patients with bipolar disorder display greater ventral striatal activation and reduced activation in the dorsolateral prefrontal cortex, explaining their preference for immediate gratification over achieving long-term goals.⁴⁰ By contrast, patients with unipolar depression exhibit an increased activation of the amygdala, which is related to rumination about negative situations but not positive situations.⁴⁰ Resilience is also positively correlated with reward dependence, which is characterized by sensitive reactions to compensation signals of achievement.³¹ Thus, the PBD group would immediately respond to the successful completion of basic military training, whereas the PUD group would ruminate about the adversities faced during the training rather than feeling a sense of achievement. However, further study must determine whether the immediately enhanced resilience of people with PBD can be maintained because this study observed participants for only 5 weeks.

This study has several limitations. First, because self-report questionnaires were used to screen for bipolar disorder in this study, participants had only PBD rather than diagnosed bipolar disorder. Therefore, the results of the present study cannot be generalized to people with bipolar disorder. However, the MDQ has good sensitivity for bipolar disorder, and previous studies suggested that subjects scoring above the cut-off value had PBD or bipolarity.²⁰⁴¹ Furthermore, because of we used both MDQ and CES-D score, PBD group is much closer to the true clinical subjects with probable bipolar disorder. Therefore, this result could also be significant for patients with probable bipolar disorder. Second, because all participants were male and subjects with serious psychopathology were excluded from military conscription examination, the results do not inform for resilience among females or patient with a serious mental illness. Future studies should investigate this issue within clinical subjects included patients with diagnosed bipolar disorder for all genders. Third, because the subjects of this study were all conscripts who must complete training as a compulsory part of military service, the participants' survey answers might not reflect their autonomous responses. Therefore, opinions about how the subjects think about basic military training itself could influence the interpretation of improved resilience, which is the main result of this study. In further studies, it will be necessary to investigate this question using structural scales.

Despite these limitations, this study was the first to prospectively investigate the positive aspect of strengthening the resilience of males with PBD and to comparing it with that of males with PUD. Temperaments, characteristics and neurocognitions, which people with PBD is a more resilience than PUB, might have more improved resilience on males with PBD than PUD, after the training. The resilience of people with PBD could be more strengthened than that of people with PUD exposed to the same environment after training even after confounding factors such as impulsivity, age and level of education are controlled. Resilience is associated with prognosis, including number of depressive episodes and quality of life on psychiatry.¹⁰⁴²⁴³ Furthermore, resilience might be an important factor in preventing the transmission of risk of the onset of bipolar disorder.⁴⁴ Finally, the present study indicates that interventions that strengthen resilience need to be developed for people with PBD and that such interventions are less effective for people with PUD.