Violence against animals has been documented to be a warning sign of potential violence against humans.
See the article at http://en.wikipedia.org/wiki/Zoosadism
It is stated there that "Studies have shown that individuals who enjoy or are willing to inflict harm on animals are more likely to do so to humans. One of the known warning signs of certain psychopathologies, including antisocial personality disorder, is a history of torturing pets and small animals.
According to the New York Times:
'the FBI has found that a history of cruelty to animals is one of the traits that regularly appears in its computer records of serial rapists and murderers, and the standard diagnostic and treatment manual for psychiatric and emotional disorders lists cruelty to animals as a diagnostic criterion for conduct disorders.'"
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