Jack Levin, author of the 2008 book, “Serial Killers and Sadistic Mur­ders — Up Close and Per­sonal,” said he found that more than half of serial killers he studied had abused ani­mals, many of them max­i­mizing the suf­fering of dogs and cats.

He did a study com­paring 150 animal abusers to those who never abused ani­mals, he said. “The animal abusers were five times more likely to commit human vio­lence, such as rape and rob­bery. They were also four times more like to commit prop­erty offenses.”

The common belief is people “grad­uate” from ani­mals to people, he said, “but we also found they may start with people and end with ani­mals — or do both at the same time.”

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