Editors' Summary Background Suicide is a leading cause of death worldwide. Every 40 seconds, someone somewhere commits suicide. Over a year, this adds up to about 1 million self-inflicted deaths. In the USA, for example, where suicide is the 11th leading cause of death, more than 30,000 people commit suicide every year. The figures for nonfatal suicidal behavior (suicidal thoughts or ideation, suicide planning, and suicide attempts) are even more shocking. Globally, suicide attempts, for example, are estimated to be 20 times as frequent as completed suicides. Risk factors for nonfatal suicidal behaviors and for suicide include depression and other mental disorders, alcohol or drug abuse, stressful life events, a family history of suicide, and having a friend or relative commit suicide. Importantly, nonfatal suicidal behaviors are powerful predictors of subsequent suicide deaths so individuals who talk about killing themselves must always be taken seriously and given as much help as possible by friends, relatives, and mental-health professionals. Why Was This Study Done? Experts believe that it might be possible to find ways to decrease suicide rates by answering three questions. First, which individual mental disorders are predictive of nonfatal suicidal behaviors? Although previous studies have reported that virtually all mental disorders are associated with an increased risk of suicidal behaviors, people often have two or more mental disorders (“comorbidity”), so many of these associations may reflect the effects of only a few disorders. Second, do some mental disorders predict suicidal ideation whereas others predict who will act on these thoughts? Finally, are the associations between mental disorders and suicidal behavior similar in developed countries (where most studies have been done) and in developing countries? By answering these questions, it should be possible to improve the screening, clinical risk assessment, and treatment of suicide around the world. Thus, in this study, the researchers undertake a cross-national analysis of the associations among mental disorders (as defined by the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition [DSM-IV]) and nonfatal suicidal behaviors. What Did the Researchers Do and Find? The researchers collected and analyzed data on the lifetime presence and age-of-onset of mental disorders and of nonfatal suicidal behaviors in structured interviews with nearly 110,000 participants from 21 countries (part of the World Health Organization's World Mental Health Survey Initiative). The lifetime presence of each of the 16 disorders considered (mood disorders such as depression; anxiety disorders such as post-traumatic stress disorder [PTSD]; impulse-control disorders such as attention deficit/hyperactivity disorder; and substance misuse) predicted first suicide attempts in both developed and developing countries. However, the increased risk of a suicide attempt associated with each disorder varied. So, for example, in developed countries, after controlling for comorbid mental disorders, major depression increased the risk of a suicide attempt 3-fold but drug abuse/dependency increased the risk only 2-fold. Similarly, although the strongest predictors of suicide attempts in developed countries were mood disorders, in developing countries the strongest predictors were impulse-control disorders, substance misuse disorders, and PTSD. Other analyses indicate that mental disorders were generally more predictive of the onset of suicidal thoughts than of suicide plans and attempts, but that anxiety and poor impulse-control disorders were the strongest predictors of suicide attempts in both developed and developing countries. What Do These Findings Mean? Although this study has several limitations—for example, it relies on retrospective self-reports by study participants—its findings nevertheless provide a more detailed understanding of the associations between mental disorders and subsequent suicidal behaviors than previously available. In particular, its findings reveal that a wide range of individual mental disorders increase the chances of an individual thinking about suicide in both developed and developing countries and provide new information about the mental disorders that predict which people with suicidal ideas will act on such thoughts. However, the findings also show that only half of people who have seriously considered killing themselves have a mental disorder. Thus although future suicide prevention efforts should include a focus on screening and treating mental disorders, ways must also be found to identify the many people without mental disorders who are at risk of suicidal behaviors. Additional Information Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000123. The US National Institute of Mental Health provides information about suicide in the US: statistics and prevention The UK National Health Service provides information about suicide, including statistics about suicide in the UK and links to other resources The World Health Organization provides global statistics about suicide and information on suicide prevention MedlinePlus provides links to further information and advice about suicide and about mental health (in English and Spanish) Further details about the World Mental Health Survey Initiative and about DSM-IV are available Background Suicide is a leading cause of death worldwide. Mental disorders are among the strongest predictors of suicide; however, little is known about which disorders are uniquely predictive of suicidal behavior, the extent to which disorders predict suicide attempts beyond their association with suicidal thoughts, and whether these associations are similar across developed and developing countries. This study was designed to test each of these questions with a focus on nonfatal suicide attempts. Methods and Findings Data on the lifetime presence and age-of-onset of Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) mental disorders and nonfatal suicidal behaviors were collected via structured face-to-face interviews with 108,664 respondents from 21 countries participating in the WHO World Mental Health Surveys. The results show that each lifetime disorder examined significantly predicts the subsequent first onset of suicide attempt (odds ratios [ORs] = 2.9–8.9). After controlling for comorbidity, these associations decreased substantially (ORs = 1.5–5.6) but remained significant in most cases. Overall, mental disorders were equally predictive in developed and developing countries, with a key difference being that the strongest predictors of suicide attempts in developed countries were mood disorders, whereas in developing countries impulse-control, substance use, and post-traumatic stress disorders were most predictive. Disaggregation of the associations between mental disorders and nonfatal suicide attempts showed that these associations are largely due to disorders predicting the onset of suicidal thoughts rather than predicting progression from thoughts to attempts. In the few instances where mental disorders predicted the transition from suicidal thoughts to attempts, the significant disorders are characterized by anxiety and poor impulse-control. The limitations of this study include the use of retrospective self-reports of lifetime occurrence and age-of-onset of mental disorders and suicidal behaviors, as well as the narrow focus on mental disorders as predictors of nonfatal suicidal behaviors, each of which must be addressed in future studies. Conclusions This study found that a wide range of mental disorders increased the odds of experiencing suicide ideation. However, after controlling for psychiatric comorbidity, only disorders characterized by anxiety and poor impulse-control predict which people with suicide ideation act on such thoughts. These findings provide a more fine-grained understanding of the associations between mental disorders and subsequent suicidal behavior than previously available and indicate that mental disorders predict suicidal behaviors similarly in both developed and developing countries. Future research is needed to delineate the mechanisms through which people come to think about suicide and subsequently progress from ideation to attempts. Please see later in the article for Editors' Summary Using data from over 100,000 individuals in 21 countries participating in the WHO World Mental Health Surveys, Matthew Nock and colleagues investigate which mental health disorders increase the odds of experiencing suicidal thoughts and actual suicide attempts, and how these relationships differ across developed and developing countries.