Several noteworthy findings were revealed in this study. The finding that approximately half of those who seriously consider killing themselves, and more than half of those making a suicide attempt have a prior mental disorder extends earlier findings from psychological autopsy studies 
and studies using clinical samples 
that have reported that most suicide attempters have a diagnosable mental disorder. Notably, the rates of mental disorder in the current study—even among suicide attempters—were much lower than those documented in prior studies among clinical samples and those dying by suicide, which suggests that the rate of mental disorders among suicidal people in the general population is lower than in these other groups. Moreover, we examined lifetime mental disorders, whereas studies of suicide decedents and clinical samples assess current or recent disorders, making the differences between clinical and general population studies even more pronounced.
Our finding of a lower rate of mental disorders among suicide attempters in the general population is consistent with other recent results indicating that prevalence estimates for suicide among those with affective disorders differ on the basis of the treatment setting from which participants are sampled (i.e., highest risk among inpatients, lower risk among outpatients, and lowest risk among the nonaffectively ill) 
. The current study extends this finding to other diagnoses. It also is notable in the current study that the rate of mental disorders was slightly, but consistently, higher among suicidal people in developed than developing countries. However, rates of mental disorders were similarly lower among nonsuicidal people from developing countries (see Tables S1
), suggesting that there is a difference in the rate of mental disorders in developed versus developing countries 
, rather than in the association between disorders and suicidal behavior.
The presence of each mental disorder examined was associated with increased odds of a subsequent suicide attempt. These results extend findings from prior studies 
by showing that mental disorders in general are similarly predictive of suicide attempts in developed and developing countries when similar measurement methods are used cross-nationally. We also found that while the ORs of virtually all temporally primary mental disorders predict subsequent nonfatal suicide attempts, these ORs vary significantly across disorders, demonstrating that type
of disorder makes a difference. In addition, we found that the relative magnitudes of different types of disorders vary across stage of progression to suicide attempts (i.e., in predicting onset of ideation in the total sample, onset of plans among lifetime ideators, onset of attempts among lifetime planners, onset of attempts among lifetime ideators who never had a plan), which means that there is no single underlying common pathway (e.g., an underlying “distress” factor) through which all disorders exert their effects on the progression to attempts.
We also presented a more comprehensive analysis of the association between comorbidity and suicidal behavior than previously available, and several aspects of this association are especially noteworthy. A strong dose-response relation was found between the number of mental disorders present and the odds of a subsequent suicide attempt, consistent with the results from several prior studies 
. In the current study, the presence of multiple disorders (i.e., “multimorbidity” 
) yielded ORs several times higher than that for any individual disorder in the prediction of suicide attempts. Despite this strong dose-response relation, subadditive interactive effects were observed, suggesting that decay exists in the predictive power of comorbidity as the number of comorbidities increases. This finding raises the important possibility for intervention planning purposes that the impact on reduced suicidality of successfully treating a single disorder such as major depression or panic disorder will be greater among people with pure than comorbid disorders. It also implies that success in reducing suicidality by treating people with high comorbidity will require treating multiple comorbid disorders rather than only a single component disorder.
A top priority for future research should be to understand why
the joint predictive effects of comorbid disorders on suicidality are as powerful as they are as well as why these associations are subadditive. Prior research has shown that high comorbidity is associated with high levels of psychological distress, impairment, and disease burden 
, but has shed little light on the dynamics of these associations. One possibility that has been suggested in the literature is that high comorbidity creates an especially intolerable situation from which people attempt to escape via suicide 
. However, if this were the case we would expect to find that the joint effects of comorbid conditions are supra-additive (i.e., the proverbial straw that broke the camel's back) rather than subadditive. Another possibility, as noted above in the discussion of limitations, is that we misinterpreted disorder complexity or severity as evidence of comorbidity. Future analyses examining the influence of distress, impairment, and burden on suicidal behavior should be conducted to investigate the effects of pure and comorbid disorders on these intermediate outcomes directly in order to help sort out alternative interpretations. Illumination of the psychological mechanisms through which comorbid mental disorders increase the risk of suicidal behavior is sorely needed in order to explain the causal mechanisms leading to suicide 
Even after accounting for the effects of comorbidity, however, each disorder considered alone continued to significantly predict subsequent suicide attempts. There has been debate regarding the extent to which some disorders appear to be associated with suicidal behavior only because they co-occur with other disorders that are truly associated with suicidal behavior. For instance, prior studies of the association between panic disorder and suicide attempts have yielded different conclusions depending on which comorbid disorders were controlled in each study 
. We controlled for comorbidity more rigorously than any prior studies of which we are aware, and despite these controls panic disorder continued to significantly predict suicide attempts cross-nationally (ORs
2.3–3.0), suggesting that panic disorder is indeed uniquely related to suicide attempts.
Consistent with results from prior studies, mood disorders—particularly major depression and bipolar disorder—were significant predictors of suicide attempts with virtually the same ORs in both developed and developing countries. Interestingly, however, several other disorders were even stronger predictors of suicide attempts in developing countries, including: conduct disorder, oppositional defiant disorder, intermittent explosive disorder, drug and alcohol abuse, and PTSD. The strong relation between impulse-control disorders and suicide attempts in developing countries extends results from a prior study using a subset of these WMH countries 
. The strong association between PTSD and suicide attempts in developing countries is intriguing. Indeed, PTSD was the strongest predictor of suicide attempts in developing countries and was among the three strongest predictors in developed countries. A few prior studies have noted a link between PTSD and suicidal behavior 
; however, PTSD has received much less attention than mood and substance use disorders as a risk factor for suicidal behavior. This finding may be due to the lower base rate of PTSD in the population, even in developing countries and those in which war exposure is more common 
, which makes this association more difficult to detect in smaller studies. Future studies should investigate whether certain types of trauma, or characteristics of traumatic events, might be particularly important in predicting suicidal behavior. Overall, these findings underscore the importance of this broader range of disorders in the onset of suicide attempts. A crucial goal for future research is to better understand how and why such a diverse range of disorders are uniquely associated with suicide attempts.
As a first step toward doing so, we found that mental disorders are differentially associated with suicidal behavior depending on which part of the pathway to suicide was being predicted. Specifically, each disorder is significantly associated with the subsequent onset of suicide ideation. However, disorders were much less useful in predicting which people with suicide ideation progress to suicide plans and attempts, and many disorders were not predictive of such progressions at all. Perhaps most surprisingly, although depression has repeatedly been shown to be among the strongest predictors of suicide attempts 
and was similarly predictive in the current study, decomposition of this association revealed that it is due largely to depression predicting the onset of suicide ideation. A diagnosis of major depression is much less useful in predicting which people with suicide ideation go on to make suicide plans or attempts, and it is nonsignificantly associated with unplanned attempts in both developed and developing countries. In contrast, disorders characterized by anxiety (especially PTSD) and poor impulse-control (especially bipolar, conduct, and substance use disorders) emerged as the strongest predictors of which ideators make suicide plans and attempts, and were especially useful in the prediction of unplanned attempts. These findings do not suggest that depression is unimportant in the prediction of suicidal behavior—indeed many suicide attempts occur in the context of a depressive episode—but only that a diagnosis of depression is not especially useful in determining who is likely to act on their suicidal thoughts. Several theoretical models have proposed that some disorders such as depression lead people to desire
suicide, and other disorders characterized by anxiety/agitation and problems with impulse-control increase the likelihood that people act
on such thoughts 
. The current findings provide support for such a model and show that this pattern of associations is consistent cross-nationally. Future research is needed to investigate whether it is indeed the impulsiveness, aggressiveness, and agitation associated with these disorders that may lead to suicidal behavior—as has been suggested in prior studies 
—or if some other aspects of these disorders account for the observed relations. This can be tested by examining the associations between specific symptoms of these disorders and suicidal behavior, and by testing whether the observed effects are mediated by the psychological characteristics proposed above (i.e., impulsiveness, etc.).
These results must be interpreted in light of several key limitations. First, although the WMH achieved an acceptable response rate overall, response rates varied cross-nationally. Differential response was controlled for using poststratification adjustments, but it remains possible that response rates were related to the presence of suicidal behaviors or mental disorders, which could have biased cross-national comparisons. A related limitation is that although data are from large representative samples in 21 countries, several of the samples were not nationally representative, and the WMH countries represent only a small sample from around the globe. Each of these factors limits the generality of the results, which should not be considered global estimates.
Second, these data are based on retrospective self-reports of the occurrence and timing of mental disorders and suicidal behavior, which introduce potential problems with underreporting and biased recall 
. Recall bias is likely to be greatest for history of disorders typically diagnosed in childhood (e.g., Conduct Disorder, Oppositional Defiant Disorder, Attention Deficit Hyperactivity Disorder), which we attempted to address by limiting the diagnosis of such disorders to those <44 y old at the time of interview; however, this does not fully negate this concern. It also is possible that the extent of recall bias present differs across suicide ideation, plans, and attempts, which could have influenced the study results. On balance, systematic reviews have suggested that people can recall past experiences with sufficient accuracy to provide valuable information 
and that such data are especially useful when prospective data are not available 
, as in the current case. In addition, the WMH clinical reappraisal studies, as noted in the section on measures, found good concordance between diagnoses based on the survey responses and diagnoses based on blinded clinical follow-up interviews 
. However, this evidence is not convincing in light of the fact that the same recall bias could occur in clinical interviews. The clinical reappraisal study did not assess the concordance between survey measures of comorbidity and assessments of comorbidity in the clinical follow-up interviews. In addition, all clinical reappraisal studies were carried out in developed countries. Parallel clinical reappraisal studies need to be carried out in the future in developing countries. An additional related limitation is that cultural factors may have influenced willingness to report the presence of mental disorders or suicidal behavior 
as well as the interpretation of survey items about these constructs 
Third, although we assessed a broad range of disorders and various aspects of suicidal behavior, we did not consider the severity, complexity, or chronicity of each disorder. This absence could have led to an underestimate of the strength of associations between mental disorders and suicidal behavior. For instance, it may be that clinical severity and chronicity are important in predicting the transition from suicide ideation to attempts. This same problem could have led to an overestimate of comorbidity to the extent that disorder complexity or severity was incorrectly characterized as comorbidity in the CIDI. It is relevant in this regard that fully structured lay-administered diagnostic interviews like the CIDI are likely to be less capable of distinguishing between secondary symptoms of complex disorders and true comorbidity, making it likely that comorbidity is overdiagnosed and some of the presumed predictive effects of comorbidity are actually effects of disorder complexity or severity. This presumably explains why a number of respondents were found to meet criteria for six or more disorders, although the focus on lifetime hierarchy-free diagnoses also contributed to this evidence for high comorbidity. A related problem is that several disorders known to be associated with suicidal behavior were not examined in this study, such as nonaffective psychosis and personality disorders. The investigation of these and other factors remain key directions for future research.
Fourth, our analysis of comorbidity was restricted to a small number of very simple models. It is possible that more subtle interactions exist among disorders, which could be uncovered in a comprehensive analysis. Given the enormous number of logically possible interactions among the disorders considered here, it might be useful for future research to use data mining methods such as classification and regression tree analysis and random forest analysis to search for consistent patterns among these disorders 
. Data mining has previously been used successfully to study the joint interactive effects of a wide range of risk factors for suicide attempts in a clinical sample 
. Given the high risk of overfitting the data with methods of this sort, though, it is important that future use of these methods in the WMH data include cross-validation in random subsamples. It will also be important for such future analyses to go beyond the simple dichotomous comparison of developed versus developing countries and to evaluate the consistency of country-specific associations as well as explore plausible interpretations of meaningful cross-national differences in patterns of association.
Despite these limitations, the results of this study have important implications for scientific, clinical, and policy efforts aimed at suicide prevention. Scientifically, this study documents the importance of controlling for comorbidity and of carefully considering which suicidal behavior is being predicted in future studies. Efforts to more carefully and precisely operationalize both the independent and dependent variables in studies of suicidal behavior will yield a clearer understanding of how and why such behaviors occur. Clinically, these results demonstrate the importance of considering not only depression but also the full range of mental disorders when evaluating patients' risk for suicidal behavior. Given the especially strong associations between multimorbidity and suicidal behavior, clinicians should always conduct a suicide risk assessment among patients presenting with multiple mental disorders.
From a public health perspective, the strong and consistent associations between mental disorders and suicidal behavior suggest that suicide prevention efforts should include a focus on screening and treating mental disorders in both developed and developing countries. Resources devoted to the treatment of mental disorders in general, and suicidal behavior in particular, currently are lacking in many developing countries 
. Our results suggest that if there is in fact a causal relation between mental disorders and suicide attempts, allocating sufficient resources to decrease mental disorders will lead to a significant reduction in suicidal behavior. Although it is not yet known whether a causal association exists, there is some evidence that efforts to decrease the occurrence of mental disorders can positively affect the suicide rate. For instance, some programs designed to train primary care physicians in the recognition and treatment of depression have demonstrated reductions in the suicide rate 
; however, not all such programs have yielded positive effects 
. Importantly, though, our results also indicate that a sizeable minority of respondents report suicidal behaviors in the absence of any
mental disorder. Thus, focusing solely on those with mental disorders is likely to miss a fairly large segment of those who engage in suicidal behavior. Identifying this subgroup is likely to be more challenging given that public health programs that screen for the presence of mental disorders will not identify these respondents, highlighting the need for novel methods of identifying those at-risk that do not rely on the presence of mental disorders 
. There is still much we do not know about how mental disorders and other factors increase the risk of suicidal behavior; however, the results of this and related studies highlight several important directions for improving the understanding, prediction, and prevention of these dangerous outcomes.