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The Macdonald triad (also known as the triad of sociopathy or the homicidal triad) is a set of three behavioral characteristics that has been suggested, if all three or any combination of two, are present together, to be predictive of or associated with later violent tendencies, particularly with relation to serial offenses. The triad was first proposed by psychiatrist J.M. Macdonald in "The Threat to Kill", a 1963 paper in the American Journal of Psychiatry. Small-scale studies conducted by psychiatrists Daniel Hellman and Nathan Blackman, and then FBI agents John E. Douglas and Robert K. Ressler along with Dr. Ann Burgess, claimed substantial evidence for the association of these childhood patterns with later predator behavior. Although it remains an influential and widely taught theory, subsequent research has generally not validated this line of thinking.
The triad links cruelty to animals, obsession with fire setting, and persistent bedwetting past a certain age, to violent behaviors, particularly homicidal behavior and sexually predatory behavior. However, other studies claim to have not found statistically significant links between the triad and violent offenders.
Further studies have suggested that these behaviors are actually more linked to childhood experience of parental neglect, brutality or abuse. Some argue this in turn results in "homicidal proneness". The 'triad' concept as a particular combination of behaviors linked to violence may not have any particular validity - it has been called an urban legend.
In Singer and Hensley (2004), firesetting is theorized to be a less severe or first shot at releasing aggression. Extensive periods of humiliation have been found to be present in the childhoods of several adult serial killers. These repetitive episodes of humiliation can lead to feelings of frustration and anger, which need to somehow be released in order to return to a normal state of self-worth. However, the triad combination has been questioned in this regard also, and a review has suggested that this behavior is just one that can occur in the context of childhood antisocial behavior and isn't necessarily predictive of later violence.
FBI Special Agent Alan Brantly believed that some offenders kill animals as a rehearsal for killing human victims. Cruelty to animals is mainly used to vent frustration and anger the same way firesetting is. Extensive amounts of humiliation were also found in the childhoods of children who engaged in acts of cruelty to animals. During childhood, serial killers could not retaliate towards those who caused them humiliation, so they chose animals because they [animals] were viewed as weak and vulnerable. Future victim selection is already in the process at a young age. Studies have found that those who engaged in childhood acts of cruelty to animals used the same method of killing on their human victims as they did on their animal victims.
Wright and Hensley (2003) named three recurring themes in their study of five cases of serial murderers: As children, they vented their frustrations because the person causing them anger or humiliation was too powerful to take down; they felt as if they regained some control and power over their lives through the torture and killing of the animals; they gained the power and control they needed to cause pain and suffering of a weaker, more vulnerable animal – escalating to humans in the future.
In a study of 45 male prison inmates who were deemed violent offenders, McClellan (2003) found that 56% admitted to having committed acts of violence against animals. It was also found that children who abused animals were more often the victims of parental abuse than children who did not abuse animals. As previously stated, animal cruelty was a way for the children to feel as if they were retaliating against those who abused, frustrated, or humiliated them.
In a 2004 study, which considered not one-off events but patterns of repeat violence, Tallichet and Hensley found a link between repeated animal cruelty and violence against humans. They examined prisoners in maximum or medium security prisons.  However, over-generalizing possible links between animal violence and human violence can have unwanted consequences such as detracting focus from other possible predictors or causes.
The idea that bedwetting has anything to do with psychological maladjustment, and certainly with later antisocial or violent tendencies, or plays some part in a triad of predictors, has been described as a destructive myth entirely discredited. Crime researchers acknowledge that it is not linked with later sociopathic behavior. It is not even clear that it is necessarily associated with distress.
However, some authors continue to speculate that enuresis may be related to firesetting and animal cruelty in some way. One argument is that because persistent bed-wetting beyond the age of five can be humiliating for a child, especially if he or she is belittled by a parental figure or other adult as a result, this could cause the child to use firesetting or cruelty to animals as an outlet for his or her frustration. Enuresis is an "unconscious, involuntary, and nonviolent act and therefore linking it to violent crime is more problematic than doing so with animal cruelty or firesetting".
According to Douglas and his fellow researchers, the triad behaviors are not causal when examining a relationship with later predatory behavior, but rather, are predictive of an increased likelihood of the future behavior patterns, and give professionals a chance to halt some patterns before they progress.