Research shows that risk factors for suicide include:Depression and other mental disorders, or a substance-abuse disorder (often in combination with other mental disorders). More than 90 percent of people who die by suicide have these risk factors Prior suicide attempt Family history of mental disorder or substance abuse Family history of suicide Family violence, including physical or sexual abuse Firearms in the home, the method used in more than half of suicides Incarceration Exposure to the suicidal behavior of others, such as family members, peers, or media figures However, suicide and suicidal behavior are not normal responses to stress; many people have these risk factors, but are not suicidal
Research also shows that the risk for suicide is associated with changes in brain chemicals called neurotransmitters, including serotonin. Decreased levels of serotonin have been found in people with depression, impulsive disorders, and a history of suicide attempts, and in the brains of suicide victims.
This answer is based on source information from the National Institute of Mental Health.