The present study investigated the predicted effects of depressive symptoms and impulsiveness on suicidal ideation. First, the results revealed that suicidal ideation significantly differentiated depressive symptoms and impulsiveness scores. Specifically, individuals with suicidal ideation scored higher than those who did not have suicidal ideation both on depressive symptoms and on impulsiveness. This agrees with the results of previous studies, although many of them focused on suicide attempts, depressive disorders, and impulsive disorders.
In terms of depression, individuals with disorders primarily commit suicide to reduce their pain, while those who are not psychopathic commit suicide for impulsiveness and to threaten others (8). Therefore, similar correlations of depressive disorder and depressive symptoms with suicide may have different mechanisms. Some previous studies verify this point of view. Kim’s study on impulsive suicide and non-impulsive suicide demonstrated that MDD is less frequent in impulsive suicide (35). In Wei’s results, individuals characterized by impulsive suicide have a lower prevalence of psychopathy (33). Another study concluded that suicide attempters without psychopathy exhibit more impulsivity than those with other disorders (53). Depressive symptoms that are subclinical are related to impulsiveness more closely than to depressive disorder. Therefore, the present study sampled a generally healthy group. These results demonstrate that depressive symptoms are positively correlated with impulsiveness and that both are correlated with suicide. Furthermore, based on these results, depression and impulsiveness are both predictors for suicide, while depressive symptoms were additionally influenced by impulsiveness.
Previous studies have examined many aspects of the connotation and extension of impulsiveness. An impulsive personality with aggression (5, 26) is thought to be the reason for suicidal behaviours, while attention deficit (ADHD) (26) and impulse control disorders (29, 30) are also risk factors for suicide. In the present study, impulsiveness consisted of three dimensions: non-planning, motor and attention. All three dimensions indirectly influenced suicidal ideation through depressive symptoms. Motor impulsiveness refers to those who do not control their behaviours (34). This agrees with studies on impulse control disorders, although motor impulsiveness is a component of personality. Attention impulsiveness means that one cannot continue to pay attention to specific stimuli or events (34), which is similar to results from studies on ADHD. Although motor and attention impulsiveness are still far from psychopathy or other disorders, impulsive personality is related to impulsive-related disorders and thus is associated with suicidal behaviour. Prior to mental and behavioural impairment, an impulsive personality is already associated with suicidal ideation. Moreover, non-planning is described as a trait that one cannot plan rationally before behaviours (34). This means that one acts without deep thought. Therefore, non-planning is more likely classified as a trait of thinking. The present study’s result that non-planning indirectly affects suicidal ideation without a direct effect indicates that it results in depressive symptoms first, followed by suicidal ideation. This conclusion coincides with the research result that impulsiveness makes people more likely to be exposed to stress or negative events and thus results in suicide (25).
Regarding suicide attempts, it is relatively easy to discriminate positive and negative external behaviours. The operational definition of the severity of a suicide attempt varies in different studies, including the times of suicide behaviours (9), medical seriousness (5), prevalence (7) and lethality (28). It is difficult to include these aspects into a single component. There is research indicating that impulsiveness does not predict the severity of suicide attempts (5). The present study considered suicidal ideation an outcome, which was a latent variable and needed to be tested with a scale. As a continuous variable, suicidal ideation severity is measured by scale scores. These results demonstrate that depressive symptoms and impulsiveness were both positively correlated with suicidal ideation. This means that these two factors positively predict the severity of suicidal ideation.
In the present study, we analysed three variables together with a correlation matrix and regression method, and all of the variables were included. In many previous studies, one of the variables was regarded as a group (36, 54–57). A predictive model built to include impulsive personality, depressive symptoms and suicidal ideation shows that impulsive personality and depressive symptoms can explain the variance of suicidal ideation independently, but this model does not mention the correlation between impulsiveness and depressive symptoms (58). Based on our result that suicidal ideation was more highly correlated with depressive symptoms than was impulsiveness (including the three dimensions), while the correlations between depressive symptoms and impulsiveness (including the three dimensions) were relatively higher than those between suicidal ideation and impulsiveness (including the three dimensions), we deduced that impulsiveness has a stronger indirect effect on suicidal ideation than direct impulsiveness. The results of regression models indicated that for total impulsiveness, including non-planning, motor and attention impulsiveness, depressive symptoms were mediators of suicidal ideation. These results are in agreement with the “stress vulnerability model” theory posited by Bonner and Rich in 1987 (59). According to this model, suicide is affected by both vulnerability (personality, emotion, cognition, etc.) (60) and stress (life events, environment, etc.) at the same time. These two factors existing together increases the risk of suicidal ideation. In previous studies, personality, especially neuroticism, was definitely vulnerable (61), while depression was regarded as emotional vulnerability (62). However, in the present study, depressive symptoms were set as a stressor because they were more a state than a trait. The mediation model demonstrated that impulsiveness was vulnerable not only to suicide but also to depressive symptoms. All impulsiveness dimensions influenced suicidal ideation indirectly. However, the results could still not define whether the inner mechanisms of the three dimensions’ effects were the same.
To confirm the causal relationships between impulsiveness and the growth of depressive symptoms as well as suicidal ideation, a longitudinal analysis was conducted followed by a cross-sectional mediation effect analysis. The results showed that impulsiveness had an indirect effect on the growth rate of suicidal ideation through the initial level and growth rate of depressive symptoms. The paths in the LGCM (Fig. 2(A)) indicated that impulsiveness directly and positively affected the growth of suicidal ideation. At the same time, indirect and negative predicting paths existed from impulsiveness to the growth of suicidal ideation. Specifically, increased impulsiveness predicted a higher initial level of depressive symptoms and a more rapid decline in suicidal ideation. At the same time, impulsiveness predicted a faster decrease in suicidal ideation through a slower increase in depressive symptoms. As a result, impulsiveness indirectly negatively influenced the growth of suicidal ideation. Because the indirect negative effect was larger than the direct positive effect, the total effect of impulsiveness on suicidal ideation was negative. Similar effects were observed for non-planning and attention impulsiveness. The total effect of motor function on suicidal ideation was not significant. This might indicate that the inner mechanisms of the three kinds of impulsiveness’ effects on suicidal ideation differ. In the present study, the severity of suicidal ideation decreased as a whole across the three waves, declining more quickly for individuals with stronger impulsiveness. This indicates that personality impulsiveness may result in inconsistent risk for suicidal ideation variance over a short time. It is difficult to detect a quickly changing state. Therefore, the impulsiveness personality criterion provides a relatively stable observed indicator. However, this study does not provide increasing suicidal ideation data, which may be more meaningful and evidential as a reference for prevention.
Despite the supported hypothesis, this study still has some limitations. First, there were only three waves in the longitudinal study, which met the lowest criteria and might have conveyed inconsistency in the results. Second, although some of the paths were significant, the effect sizes were relatively low (below 0.3). Third, the participants included in the samples were limited to college students, so the effect is uncertain in other demographics. Therefore, future studies need to include more waves in the longitudinal analyses to increase the statistical consistency. More samples from a variety of groups, such as middle school students, adults, and elderly individuals, should also be included. Moreover, other methods, such as electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI), can be administered to verify the indirect effects more precisely and to identify the mechanisms among them.