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Self-injury has been used pseudo-synonymously in the psychological literature with terms such as self-harm and self-mutilation to encompass a broad spectrum of behaviors from suicide to self-administered non-lethal bodily harm to culturally motivated body modification. Traditionally broad categorizations have complicated the study of the motivations, causes and purpose of the various forms self-injury may take. It is now accepted that suicide, acute, highly destructive self-injuries (such as ocular enucleation or self-castration), culturally motivated body modifications and superficial/moderate self-injurious behaviors (SIB, such as head-banging, citing, cutting, or hair pulling) are distinct behaviors associated with different motivations. Here we discuss SIB, which can be defined as “the deliberate, direct destruction or alteration of body tissue without conscious suicidal intent” (Favazza 1998), and may have the most direct parallels with observations in non-human primates and other mammals. In humans, although these behaviors are comorbid with a number of psychiatric conditions and a history of sexual or physical abuse during childhood, mild SIB are also common in typically developing children before the age of 4 (hair-pulling, head-banging) and healthy undergraduate college students. These behaviors often become strongly entrenched and are among the most difficult to treat suggesting they are providing some value to the individual.
According to patient self-reports, the goals of these behaviors are explicitly non-suicidal and rather provide relief from intense negative affect and dissociative thoughts, and the methods frequently used for SIB (cutting, head-banging, burning, biting) support non-lethal goals. Interestingly, some of the most affective pharmacological treatments against SIB are beta-blockers and naltrexone (an opioid antagonist) which work to block the endorphin release associated with SIB, again suggesting a positive benefit for the individual. Further research has suggested that heart rate and levels of anxiety, depersonalization and tension are reduced following SIB, while euphoria and feelings of control are increased. Taken together, this evidence supports a view where SIB is a type of self-medication that provides temporary relief from distress such as anxiety, dissociative feelings and racing thoughts.
Instantiated in this way, human SIB may share a number of features, including underlying motivations, with similar behaviors observed in non-human primates and other animals. While evidence of SIB-like behavior is rare in wild populations, zoo and laboratory housed mammals show similar behaviors, such as wound-picking, head banging or self-biting, in a moderate to high prevalence. Although few careful studies exist, it is often attributed to “boredom”, “frustration,” or “stress.” Importantly, early maltreatment and stress such as isolation and repeated experimentation, are associated with increased SIB-like behaviors in laboratory macaques. There was also evidence that self-biting in macaques occurred in direct response to stress (blood draws) and was associated with a post-bite lowering of heart rate and arousal, perhaps suggesting a similar affect regulation function as proposed for humans. Although assessing intent is hard in non-human mammals, preliminary evidence may support a model whereby SIB and SIB-like behaviors are evoked by a similar desire to regulate negative affect.
When described as above, the underlying motivations of relieving negative affect and anxiety are likely shared among our closest ancestors. Human unique technology, access and use of diverse tools and rich culture gives the greatest variety of self-injurious behavior. Where non-human animals are mostly limited to using their body to inflict SIB such as, self-biting, wound-picking and head-banging, humans additionally use all manners of tools, expanding the repertoire of SIB to include cutting, burning, etc.
There are few truly cross-cultural studies, but it has been suggested that SIB are instantiated in the above way across all human cultures.
Many of the differences in the instantiation of SIB can be attributed to physiology and cultural innovations. Humans have an exponentially richer array of tools to incorporate into SIB but a relatively less potent physiology. As a result SIB in humans much more frequently includes the use of tools while maintaining similarities with non-human animals by incorporating, although to a necessarily lesser extent and effectiveness, over-grooming, self-biting, bodily-banging or hair-pulling.
It is hard to imagine self-injurious behavior as a directly selected adaptation, especially because of the various forms it takes within our own species and across animals. Speculatively, it appears to be a spandrel that may emerge from a necessity to regulate and reign in an evolved anxiety-like system, which would no doubt have been selected for.
As prevalent as SIB are in modern humans, putting them into the functional context of regulating negative affect, feelings of depersonalization and anxiety may provide avenues for therapy.
SIB has been observed in many different zoo and laboratory mammals, and may be a universal response in all mamammals. The most careful studies in Macaques have clearly demonstrated the presence of SIB and begun to probe the goals and factors that evoke these behaviors.
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