Exactly, MacDonald! Psychological studies have shown that individuals who take pleasure in inflicting harm on animals are more likely to do so to humans. One of the known warning signs of certain psychopathologies, including anti-social personality disorder, also known as psychopathic personality disorder, is a history of torturing pets and small animals, a behavior known as zoosadism. The standard diagnostic and treatment manual for psychiatric and emotional disorders lists cruelty to animals a diagnostic criterion for conduct disorders, though it should be noted that the inclusion of animal cruelty within this standard only began with DSM-IV. A survey of psychiatric patients who had repeatedly tortured dogs and cats found all of them had high levels of aggression toward people as well, including one patient who had murdered a young boy. MacDonald Triad, indicators of violent antisocial behavior in children and adolescents. According to the studies used to form this model, cruelty to animals is a common (but not with every case) behavior in children and adolescents who grow up to become serial killers and other violent criminals.
There is some research, however, in both the UK and the U.S. that challenges the assertion that animal cruelty begets human-directed violence. Heather Piper's work, published in a 2003 article in the Journal of Social Work, posits that the presumed linkage between animal cruelty and future, human directed violence might be a "sheep in wolve's clothing." Arguing that the Human Society and other animal rights organizations have popularized the seemingly "common-sense" claim that cruelty to animals is a sort of practice for later cruelty toward humans, Lea and Stock, in a Spring 2007 Proteus: A Journal of Ideas article dispute the reliability and validity of the science that initially established this claim. MacDonald's work, for instance, was based upon very few cases and included no control group; other studies, similarly, looked only at the backgrounds of serial killers to make their claims. The reference earlier in this article to the "fact" that all of the psychiatric patients who, in one study, had reported abusing dogs or cats also reported feeling a high level of aggression toward humans was found at the Tulsa SPCA page. In fact, that study and others like it are dated and do not evidence the modern scientific standards of rigor such as including a control group.
Lea's book, "Delinquency and Animal Cruelty: Myths and Realities about Social Pathology," for instance, uses a community sample (not just a sample of criminals or serial killers but a more general sample of a population of twenty-something year old Americans) to explore how common animal cruelty is among a non-institutionalized, non-criminal population and finds that 22% of males report having engaged in such acts. Additionally, Arluke et al., in a 1999 article in the Journal of Interpersonal Violence, find that, while some people are first cruel to animals and later cruel to humans, others are first cruel to humans and later to animals and still others simply alternate haphazardly between both types of cruelty. Finally, there is also evidence that some serial killers find a strong affinity to animals and never express cruelty toward animals but have no such qualms about enacting cruelty toward humans (this paradigm was also demonstrated by Hitler and others in the Nazi leadership).
The "Mask of Sanity" is a book written by Hervey Cleckley, M.D., first published in 1941, describing Cleckley's clinical interviews with adult male incarcerated psychopaths. It is considered a seminal work and the most influential clinical description of psychopathy in the 20th century. The basic elements of psychopathy outlined by Cleckley are still relevant today. The title refers to the normal "mask" that conceals the mental disorder of the psychopathic person in Cleckley's conceptualization. Cleckley describes the psychopathic person as outwardly a perfect mimic of a normally functioning person, able to mask or disguise the fundamental lack of internal personality structure, an internal chaos that results in repeatedly purposeful destructive behavior, often more self-destructive than destructive to others. Despite the seemingly sincere, intelligent, even charming, external presentation, internally the psychopathic person does not have the ability to experience genuine emotions. Cleckley questions whether this mask of sanity is voluntarily assumed to intentionally hide the lack of internal structure, or if the mask hides a serious, but yet unidentified, psychiatric defect.
An expanded edition of the book was published in 1982, after the DSM, the manual used in the United States for categorizing psychiatric disorders, had changed the name and standards for the classification of psychopathy to antisocial personality disorder, incorporating most of Cleckley's 16 characteristics of a psychopath listed below. The original edition of the book is no longer available.