Abnormalities of 5-HT and noradrenergic functioning have been implicated in aggressive impulsivity, SIB, and suicidal behavior. The role of DA and GABA in human studies of these behaviors requires further investigation. Most studies suggest that impulsive aggression is related to lower levels of CNS 5-HT. Some studies demonstrate that increasing NE correlates to impulsive aggression, whereas other studies demonstrate an opposite relationship. The role of NE in impulsive aggressive behavior is still unclear. Self-injurious behavior is similar to impulsive aggression in that it seems to be mediated by the neurotransmitter systems previously mentioned. For example, the presence of lower levels of 5-HT and abnormalities in the DA system are related to SIB in patients with BPD and depression. SIB severity also seems to be influenced by neglect (e.g., severe isolation during rearing). As animal studies suggest, increasing the amount of isolation and an earlier onset of isolation increase the severity of SIB. Suicidal behaviors and the lethality of suicide attempts may also be linked to the abnormalities in neurotransmitter systems similar to those found in patients with impulsive aggression and SIB, namely, lowered 5-HT transmission and enhanced DA and NE functioning. Understanding the biological triggers of impulsive aggression or SIB may allow for the evaluation of suicidal attempts and completion from a different perspective and, in conjunction with genetic predictors, may eventually help with the early prediction and prevention of suicidal behaviors. Additional studies of live subjects and postmortem brains will assist in clarifying the neurobiology of suicidal behaviors that are common to many disorders and are clinically relevant to BPD.