Research Paper: The Function and Etiology of Self-Injuring Behavior in Human and Non-Human Primates

So back when I was a cool kid in college at UCLA, I took a class with Jared Diamond, who is pretty much the coolest guy ever. For my final paper, I wrote possibly my favorite research paper I've authored. Just rediscovered it cleaning out my folders, and hey why not share back when I could write and do school!

The Function and Etiology of Self-Injuring Behavior

in Human and Non-Human Primates

Four percent of the United States population, and up to fourteen percent of college students, performs deliberate self-injury (Klonsky 2007). Human self-injury (cutting, burning, head-banging, etc.) is a socially stigmatized subject that in the past was often thought to be associated with mentally disabled individuals. Self-injury is in fact a symptom of various mental health disorders, primarily borderline personality disorder, but also schizophrenia, post-traumatic stress disorder, and various others (Klonsky 2007). In more recent years self-injury, particularly cutting, has come to be associated with the “emo” population, angsty teenagers who are desperate for attention. However, the phenomena of self-injury is far from limited to the mentally handicapped and the transitional adolescent, and should not be written off as merely a symptom of mental disability or teen anxiety. This becomes apparent with the observation by researchers and zookeepers, first made at least eighty years ago, that captive populations of non-human primates perform various self-injurious acts just as humans do. The fact that monkeys and apes self-injure suggests that self-harm is not solely a product of human psychoses and experiences, but may have a biological basis and function. Self-injury is by no means a healthy habit, and in identifying it as possibly having partial genetic origins, I don’t mean to validate the practice. Rather, by understanding both the social and biological functions of self-injury, we have a more promising chance of finding a treatment for the root cause of self-injury, rather than just the symptoms.

Different scholars (Favazza 1987, Ross & McKay 1979, Favazza & Simeon 1995) have come up with several categories of self-injury, based on the severity and frequency of the behavior. The two broadest categories are moderate (high-frequency and low severity injury) and severe (low frequency and high severity injury) (Dellinger-Ness & Handler 2006). Severe self-injury can include the gouging out of eyes or amputation of limbs or genitalia, while moderate self-injury is characterized by less mortal wounding, such as shallow cutting or hair-pulling (Dellinger-Ness & Handler 2006). Severe self-injury is commonly associated with some form of psychosis and occurs during a manic episode, usually as a single occurrence (Favazza 1989). Moderate self-injury, on the other hand, inflicts relatively mild wounds in comparison, and is performed much more frequently. This paper will focus primarily on moderate self-injury because the cause of this behavior is not as clear as it is for severe self-injury, given that moderate behavior is less associated with psychoses.

A distinction can also be made between direct and indirect self-injury (Ross & McKay 1979). Indirect self-injury can include a wide variety of maladaptive behaviors, such as alcoholism, drug addiction, eating disorders, and so forth. These behaviors are not always intended to cause oneself harm, and injury may occur as an undesired side-effect. This paper will focus on direct self-injury, where an individual inflicts explicit harm upon themselves.

A final major distinction that is drawn between different self-injury models is that of social versus pathological/intrapersonal causation and function of self-injury. There are a wide variety of socially acceptable methods of self-injury – in our society, for example, it is common for men and women of all ages to have body piercings or tattoos, both of which are painful to receive and result in semi-permanent damage to the body. Self-mutilation is practiced, accepted, and even expected in some religions, such as Christianity, Tibetan Tantrism, North American Indian mysticism, and various forms of Shamanism (Favazza 1987). Among the Aztec, for example, autosacrifice and blood-letting were required to nourish the Gods and keep the Earth alive (Leon-Portilla & Shorris 2001). Blood-letting is also seen as a healing act in certain societies. Among aborigines from a Pacific Island in the Gulf of Carpentaria, the chest is cut open to let out a diseased rat from inside the person’s body (Favazza 1987). This practice, and various others, demonstrates the socially acceptable ritual aspects of self-injury.

Self-mutilation can also function as a social bonding experience or demonstration of power. Male castration, body scarification, or female foot-binding, for example, are physical and irreversible markers of one’s gender and class (Favazza 1987). Adolescent initiation rites, which occur in many tribal societies, frequently involve self-mutilation. Self-mutilation allows uncertain adolescents to demonstrate their strength and willpower, and also fosters bonds between those participating in the ritual. By subjecting themselves to the pain of these rites, young adults are also demonstrating their willingness to submit to authority (Favazza 1987, Morinis 1985). As we will see further in this paper, pathological human self-injurers sometimes self-harm in an attempt to fit into their society (Nock 2008). It is intriguing that the frequently socially unacceptable act of pathological self-mutilation might hold much in common with socially sanctioned rites of passage.

Pathological self-injury is much more difficult to understand than cultural self-mutilation, because at first glance it seems maladaptive and serves no “good” purpose. Given the sensitive nature of the topic, it is hard to perform lab tests on self-injurers. It is not, for example, socially acceptable for a researcher to collect a group of self-harmers and ask them to slit their wrists while he or she collects data. Methods of gathering data, therefore, are not optimal and are frequently biased. E. David Klonsky summarizes three main methods of data collection that have been used in the recent studies of pathological self-injury (Klonsky 2007). Two involve the self-reporting of self-injurers (compiled by Klonsky): in one, participants are asked their reasons for self injuring, while in the other they are asked to report the phenomenology of their experience – emotions and events that surround the episode (Klonsky 2007). These methods are clearly subjective, especially given that many of the tests ask participants to choose from a list of possible reasons or emotions surrounding their behavior rather than allowing them to come up with their own. The final method involves lab testing of self-injurers using self-injury proxies –mental envisioning of self-injuring, for example (Klonsky 2007). While a more objective method than the previous two, this approach has its flaws as well. The most glaring issue involves the use of proxies in place of true self-injury, and to what extent these “watered-down” episodes accurately represent true self-injury. Also, if self-injury occurs as a reaction to a specific event or emotion, simply reproducing the symptom in a lab does nothing to get at the trigger that would have naturally caused self-injury. Thus, there are many problems with the current methods of research; given that, however, there are still some interesting general trends that can be found among the data.

From the research examined, I encountered a wide range of models to explain the occurrence of self-injury. These explanations include: anti-dissociation, environmental causation (for example, childhood trauma), self-punishment, anti-suicide, sensation-seeking (stimulation), interpersonal boundary creation, creation of wounds which will heal, self-injury as a “friend”, interpersonal influence (attention-seeking), signaling fitness and strength, and affect-regulation (alleviating negative affects, such as high heart rate) (Dellinger-Ness & Handler 2006, Favazza 1987, Klonsky 2007, Nock 2008, Suyemoto 1998). These models present varying explanations of elicitation, etiology, and function of self-injury, and some have more evidence than others. In reality, an explanation of self-injurious behavior will probably include a variety of these models and others not presented, and may vary from person to person.

Dissociation is a depersonalization event that involves numbness and a feeling of being “unreal” and disconnected from oneself. There is empirical data that suggests a correlation between dissociative events and self-injury (Brodsky et al. 1995, Gratz et al. 2002, Zlotnick et al. 1996). Depersonalization events can be frightening, and data shows that acts of self injury can quickly end the episode, perhaps by the sight of blood or the physical sensation of pain (Favazza 1987, Gunderson 1984, Simpson 1975). 54-55% of several participant samples reported anti-dissociation or feeling-generation as a primary reason for self-injuring, citing causes such as “to stop feeling numb” or “to feel something, even if it is pain” (Brown et. al. 2002). As one participant stated, “I’d rather die than face being unreal… it’s all right to hurt yourself because it proves you are real” (Favazza 1987). This data suggests that for certain individuals, anti-dissociation functions as a trigger for self-injury. This reason does not hold, however, for all self-injurers, and in some studies a low percentage of participants reported any sort of feeling-generation or anti-dissociation cause (for example, Herpertz 1995 and Shearer 1994). However, the high percentages of anti-dissociative behavior reported in some studies suggest that anti-dissociation cannot be completely discounted as a factor of self-injury for certain individuals.

An environmental model suggests that self-injurers are raised and live in situations where they are taught to punish themselves, or learn to associate care-giving with pain (Dellinger-Ness & Handler 2006, Linehan 1993). This could also be termed the “trauma model”, because it is frequently prescribed to people who went through some sort of childhood abuse. There is no doubt that there is a link between self-injury and childhood trauma, as evident from vast quantities of data on this subject (Gratz et al. 2002, Yates 2003, Zlotnick 2996, etc.). This explanation for self-injury is more socially preferred than most others, because it gives a clear and decisive reason for such a stigmatized behavior as cutting. The blame can be placed entirely on the perpetrator of the childhood mistreatment, and the focus shifts from a minor social offense such as self-injuring to a major crime such as child sex abuse. The problem with this model is that many self-injurers were never abused as children, nor have they experienced any sort of drastic trauma (Yates & Carlson 2003). To focus entirely on a group of abuse victims ignores another population of individuals that suffer from maladaptive behavioral practices as well. It also discounts data suggesting that there must be another cause for self-injury in addition to, or in place of, environmental factors.

Another frequently cited reason for self-injury among participants is self-punishment, or flaw correction. A self-punishment model proposes that self-injury is an expression of self-hatred or self-derogation, perhaps in response to what the participant views as bad or sinful thoughts and actions. Favazza calls this a “cathartic release of anger" (Favazza 1987 p. 194). In some studies, self-punishment was strongly supported – for example, in a study by Brown et al. (2002) 63% of participants listed “self-punishment” as a reason for self-injuring. In another study of adolescents, 70% chose “I did not like myself”, 64% “I felt like a failure”, and 63% “I was angry at myself” (Laye-Gindhu & Schonert-Reichl 2005). In other studies, however, self-punishment was not strongly supported, with only about 10% choosing it as a reason (Herpertz 1995, Kumar 2004). Like dissociation, self-punishment is clearly an important aspect of self-injury to some, but not all, self-harmers.

Other models to explain self-injury have gathered less supportive data. Anti-suicide suggests that self-injurers harm themselves non-lethally to prevent actually killing themselves. It is cited by some, but not a large number, of self-injurers. In many studies it had no data, but that could be because the option was not presented for participants to chose. In studies where an anti-suicide option was presented, around 40% selected it – a number which is not insignificant, and may warrant further investigation (Nixon et. al 2002, Laye-Gindhu & Schonert-Reichl 2005).  Sensation-seeking is probably the least-supported model of self-injury, with only between 5-10% of participants endorsing an option such as “I thought it would be fun” or “to experience a ‘high’ that feels like a drug high” (Laye-Gindhu & Schonert-Reichl 2005, Osuch et all. 1999). Unlike tattooing and body piercing, or skydiving and taking other risks, most self-injury does not seem to be performed for the purpose of thrill-seeking or marking oneself as a daredevil. Finally, self-injury has been explained by creation of interpersonal boundaries, or the use of self-injury to mark the body as one’s own. This model is only modestly supported as a reason for self-injury, and only a small fraction of self-injurers report interpersonal boundaries as a reason for self-harm (Klonsky 2007). These three models may play a minor role in the self-injury of some individuals, but probably cannot function on their own as sufficient explanations for the behavior.

Self-injury and the physical wounds themselves can be seen by the wounder in a positive light, as something to care for, or even as a “friend”. Favazza describes certain patients who injure themselves so that they may care for the wound, and “nurture it to health” (Favazza 1987 p. 198). He also cites patients as enjoying the healing process that occurs after wounding: “…with a few strokes of a razor the self-cutter may unleash a symbolic process in which the sickness within is removed and the stage is set for healing as evidenced by a scar” (Favazza 1987 p. 195). As we saw in the self-punishment model, many self-injurers are unhappy with themselves, either physically or mentally, and desire healing. A mending wound is a physical representation of that wish, and a scar also provides a physical mark to remind the injurer of the occasion. Finally, self-injury is sometimes experienced by the wounder as a soothing presence, a “special friend”, when they feel cut off from the rest of the world: “This behavior is a warm, understanding friend. No, I’m not psychotic. I don’t believe this literally – more metaphoric. The behavior definitely does help me get through rough times” (Favazza 1987 p. 198). None of these positively spun models, however, have very much data backing them, nor do they seem to describe a large percentage of the self-injuring population or inform us of any sort of causation.

Most models presented to explain self-injury (outside of socially accepted self-injury) are intrapersonal in nature; there are, however, several interpersonal models that have been suggested. Matthew K. Nock has examined these models in detail, and suggests that the reason social models are so often thrown aside is because researchers do not want to augment the popular view of self-injurers as “doing it for attention” (Nock 2008). Social models are important, however, because “actions speak louder than words”, and there is evidence that self-injury does in fact have a social function at least some of the time (Nock 2008). The question is raised of why individuals would resort to self-injury in the place of words to express social desires. But as we will see later in this paper, self-injury is not limited to humans. If certain individuals did in fact inherit this maladaptive coping mechanism from our primitive ancestors, then the mechanism evolved in the absence of speech. In the animal world, “behavior as communication” is very important, and can be used to convey signals of honesty, quality, and distress (Nock 2008). The honesty of a signal is important, because if it could be faked it would be useless as a signal. The “handicap principle” applies to signals that can only be made at a cost to the signaler, a penalty that cannot be faked (Nock 2008). Stotting, for example, is a behavior performed by gazelle in which they bounce incredibly high on locked legs when running from a predator. This extra expenditure of energy signals to the predator that the individual is in good health, and it would be better worth the predator’s time to chase a different gazelle. And, in fact, predators tend to chase gazelle that are least able to stot (Caro 1986). Nock, then, applies this principle to human self-injury, suggesting that it could signal strength and fitness to others (just as athletic performance does), and in some cases ward off predators by signaling “resilience to aggression” (Nock 2008, p. 164). This signal represents an attempt to both stand out as capable but also fit into society because of that strength.

Behavioral signals can also communicate distress, especially when verbal signals are not eliciting the desired response. Nock calls this a “strategy of escalation” (see Figure 1).

Figure 1: Escalation from speaking to behavior (Nock 2008).

Speaking is a relatively low cost behavior when communicating distress. If, however, verbal signals fail to achieve the result the speaker wants, they may escalate to higher and higher cost behaviors – eventually, scratching and cutting. One aspect missing from this model is an explanation of why certain individuals escalate to the point of self-injury, while others are able to get what they want with only words. Research has begun on this problem, and initial data suggests that self-injurers are less verbally fluent than non-injurers (Photos & Nock 2006), and that they have a harder time expressing emotion (Gratz 2006). Thus, while many can find fairly low-cost methods of eliciting care-giving – talking or crying – self-injurers may find these methods insufficient, and resort to self-harm. It is interesting, however, that while this model is called a “social” model, it still relies on certain biological characteristics of the self-injuring individuals – that they are unable to express emotion, for example. This illustrates the inability to dissociate social and intrapersonal explanations for self-injury.

The final and most well-supported model for self-injury is that of affect regulation, the alleviation of negative thoughts or feeling and the initiation of arousal (heart rate) reduction. This model includes tension and stress release, thought distraction, controlling negative feelings, etc. It was strongly supported in almost every survey, with an average of about 70% of the participants listing some sort of affect-regulation reason for self-injury (Klonsky 2007). Subjects reported that they felt stressed, unhappy, anxious, etc., before self-injuring, and more relaxed and peaceful after (Klonsky 2007). Affect-regulation is also the only model that was supported in laboratory studies. Haines et al. (1995) and Brain et al. (1998) asked subjects to imagine themselves self-injuring in the labs, while various aspects of arousal such as heart rate, respiration, and skin resistance level were measured. The researchers found that, as expected, arousal was high in the period leading up to the imagined self-injury, and fell considerably during and after self-injury. Control groups, who were asked to imagine non-self-injurious stressful events such as accidentally cutting themselves with a kitchen knife, did not experience reduced arousal (Haines et al 1995, Brain et al. 1998). Self-injurers experience a physically different response to self-harm than non-self-injurers do, which could suggest a method and reason for the maintenance of the behavior.

Affect regulation is an important model for self-injurious behavior not only because it is so well supported in human studies, but because it appears to apply to nonhuman primates as well. Unlike the wide variety of explanations for human self-injury, there are few for that of nonhuman primates. Zoologists have known for a long time that primates engage in self-injuring behavior, and while there is almost no data on wild primate self-injury there is a modest amount of data on captive primates. The primary subjects of these studies have been rhesus monkeys, because they do well in laboratories and thus are used extensively for medical research (Novak 2003). Although a majority of the monkeys thrive in the labs, some do not and develop abnormal behavior. This behavior is characterized as pathological if it becomes frequent, disruptive, and dangerous to the health of the animal (Novak 2003) – three features that are characteristic of human self-injury as well. The self-injurious behavior of monkeys generally includes self-biting and hair-pulling, although more extreme cases have been documented. One monkey named Cupid, for example, “tore a three inch gash in his hip, ripped his scrotum open, lacerating and exposing one testicle, and mutilated the end of his tail (Tinklepaugh 1928 p. 298). Usually, however, monkeys labeled as self-injuring only self-bite periodically throughout the day, and rarely to the point of wounding. Monkeys, like humans, have preferred biting sites, usually on the limbs or abdomen (Novak 2003). Several studies have been done to determine the method for maintenance of self-injury, as well as explore its etiology.

Affect regulation has been proposed as a function for maintenance of self-injury, with the assumption that individuals must get some sort of benefit from harming themselves or they would not engage in such behavior. Among nonhumans, this hypothesis can be studied directly by monitoring the heart rates of self-injuring individuals before, during, and after biting episodes. Novak (2003) fitted individuals with a heart-rate monitoring vest, and found that the monkeys’ heart rates did rise dramatically before and during self-biting episodes (more so than vest-biting out of curiosity), and then dropped to below the initial rate after the biting episode (see Figure 2).

Figure 2: Heart rates in self-injuring individuals self-biting and vest-biting (Novak 2003)

This provides good evidence that self-injury does in fact have an affect regulation function in monkeys, and probably in humans as well. This lowering of the heart rate could explain why human self-injurers report feeling calmer after an episode of self-injury. There is little data on the mechanisms of affect-regulation, and how self-injury actually reduces the heart rate. Klonsky suggests that self-injury could serve as a form of distraction, although why less harmful methods of stress reduction such as exercise cannot function just as well remains a mystery (Klonsky 2007). It is also possible that there are biological effects of self-injury, such as the release of endorphins, which could “induce pleasant feelings” (Klonsky 2007 p. 236). Again, more research is needed to further explain the mechanisms of affect regulation.

While affect regulation provides a good model for the maintenance of self-injury, the etiology of this behavior is not well understood. It is, however, of great interest to researchers not only to stop the monkeys from self-injuring, but in the hope that findings could be applied to human self-injurers as well. One striking feature of almost all monkeys who self-injure is that they share the common experiences of adult isolation, abnormal rearing, and repeated experimentation (Dellinger-Ness & Handler 2006, Novak 2003). Not all monkeys who share these experiences self-injure, which underscores the fact that the behavioral maladaption is multifaceted and is not purely an environmentally produced behavior. However, it is interesting to see that environmental situations do play some role – if self-injury were only caused by brain chemistry, we would expect to see an even distribution of self-injurers throughout the population. The fact that they seem to clump around certain experiences suggests a mixture of biological and environmental causality.

Adult isolation seems to be the most common factor among self-injurers, with at least 10% of all monkeys housed in isolation developing self-injuring behavior (Novak 2003, Reinhardt & Rossell 2001). Several factors seem to play a role in the development of self-harming tendencies: the longer an individual remains isolated, the younger they are when introduced to isolation, and the larger the portion of time an individual spends in isolation per day, the more likely that individual is to develop self-injurious tendencies (Dellinger-Ness & Handler 2006). Abnormal rearing can also be an indicator of self-injuring behavior, especially if an infant is isolated after being separated from its mother (Dellinger-Ness & Handler 2006). There is evidence that early isolation changes the brain chemistry of these infants, and the longer an infant remains in isolation, the more susceptible to self-injury the individual is later in life (Kraemer et al. 1997). Even normally raised infants, however, can develop self-injuring behavior if isolated when they are older: Novak calls isolation “the single most significant predictor of self-injuring behavior” (Novak 2003 p. 10). Finally, repeated experimentation seems to be correlated with increased self-injuring behavior (see Figure 3).

Figure 3: Total number of medical procedures and blood samples (differences significant at P < 0.02) (Novak 2003)

Monkeys who were subjected to more medical procedures and blood drawings were more likely to be self-injuring. These three common experiences characterized the majority of self-injurious behavior observed among monkeys.

These primate studies are remarkable in the parallels that can be drawn between human and nonhuman results. The sensitive nature of human self-injury makes it difficult to see the larger picture behind the phenomenon; nonhuman primate studies make it easier to do so. From these data we can see that many human experiences that we associate with self-injury are not distinctly necessary for the behavior to occur. Self-injury occurs in the absence of particular human traumas such as physical or sexual child abuse. Rather, these specific events fit into a more general picture of an environment characterized by isolation and abandonment. Many aspects of human self-injury can be understood as existing within this general picture. Disassociation episodes, for example, frequently occur with the absence of loved ones (Gunderson 1984). This absence could easily stimulate feelings of loneliness and rejection in the patient. Individuals who self-injure out of a need to punish themselves may have been raised in a neglectful environment where they were taught to be harsh on themselves (Linehan 1993). Self-injury is easier to analyze when one takes a step back to see the behavior as not merely a product of unique human experiences, but as a cross-species maladaptive coping mechanism.

From the data presented, it is clear that no one model can explain the occurrence of self-injury among humans or non-human primates. Affect-regulation seems the best explanation for the maintenance of self-injurious behavior, but does not explain what causes the behavior in the first place. Environmental factors seem to play large role in determining which individuals are at higher risk for developing self-injurious behavior, but certain individuals – for example, high-reactor monkeys – seem to be more biologically at risk. Further research is needed to more fully understand what causes certain individuals to self-injure, while others can deal with the same situations in a less lethal way.

 

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