Background: Male completed suicide rates exceed those of women in many countries.About 90% of suicide victims have some underlying psychopathology, but not all people with mental disorders become suicidal.Both psychological and biological factors affect moods, and environmental factors can exert their effects through both of these.Men and women differ in the obvious chromosomal and sex hormone aspects, but what other biological factors may in part explain some of the differences observed in male to female suicidality?Method: A systematic literature search was performed in Scopus, Pubmed, and Web of Science databases.Search terms used were title: suicid*, gender, sex, all fields: difference, men, male, biolog*.Titles and abstracts of the articles found were scanned and duplicates eliminated.Relevant articles were obtained and a synthesis made.Results: Altogether 319 abstracts remained after screening for relevance and duplicates from the searches.Based on the literature investigated, the male serotonin system appears more robust and resistant to change than that of women.The male dopamine system appears more sensitive to negative effects than that in women.A high-activity MAO-A enzyme that breaks down catecholamines and serotonin appears to be a risk factor for suicide in men, while a high-activity COMT enzyme appears to protect men from suicide.Changes in testosterone level and subsequently in aggression may contribute to suicide risk, and both high and low testosterone has been implicated.Low testosterone seems to be connected to high lethality attempts, and oestrogen appears to offer protection.Low levels of neuroprotective plasticity enhancing BDNF and steroid starting material cholesterol appear to be risk factors in men.Conclusion: A more resistant serotonin system may protect men from depression, which is more common in women than in men.A less resistant dopamine system may expose men to impulsive, aggressive behaviour, which may lead to suicidal behaviour.Crudely simplified, these effects may mean that men are less likely to suffer from depression (low serotonin), but may be more sensitive to increased impulsivity and aggression (high dopamine).High activity MAO-A enzyme that breaks down catecholamines and serotonin appears to be a risk factor for suicidality in men, while a high activity COMT enzyme that does not break down serotonin but only catecholamines seems to protect men from suicide.This may imply that catecholamines dopamine, noradrenaline, and adrenaline need to be more effectively controlled in men to avoid suicidality, while serotonin may be important also but less critical.Oestrogen in general protects from suicidality and depression, and since male bodies can synthesize it continuously from testosterone, it may better protect them from suicidality.However, male oestrogen concentrations are lower than those in women, offering less protection.These biological effects may explain some of the differences between male and female suicidal behaviour, and future genomics and proteomics research will most likely offer more insight.