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Many researchers have pointed to psychological and psychiatric issues in an effort to explain for the habitual disorder of nail biting. In other words, it is suggested that nail biting might be reflective of underlying psychopathology (Dufrene et al., 2008). The etiological factors for nail biting reported in a body of research are anxiety, stress, tension, loneliness, and inactivity (Tanaka et al., 2008). Situations that trigger fear, boredom, or pain are also associated with fingernail biting (Doctor, 2000).

Since the early researches into nail biting, this habitual disorder has been

considered a reflex of emotional imbalances with documented demonstration of anxiety and tense moments (Deardoff et al., 1974). These common explanations have been strongly supported by later researchers, such as Foster (1998) and Wood et al. (2001). In addition, nail biting is widely believed to be a stress-relieving oral habit in both children and young adults (Sachan & Chaturvedi, 2012).

The causative agent in fingernail biting is often thought to be anxiety. The

relationship of nail biting and anxiety was reported in a study on primary sociopaths by Walker and Ziskind (1977) using the Cornell Medical Index Health Questionnaire. The results indicated that the incidence of nail biting in primary sociopaths (48.0%) was significantly greater (p less than .01) than the incidence of nail biting in the control group (24.0%). Some researchers have also suggested an association

between nail biting and anxiety, with evidence on the positive correlations between nail biting and the Taylor Manifest Anxiety Scale (Klatte & Deardorff, 1981).

In a study in 1988, interviews were conducted with 24 teachers, and 32 parents about children who had nail biting, thumb sucking and motor stereotyped

behaviours (Foster, 1998). It was concluded that nail biting behaviour appears to be a reflection of mood, such as nervousness (Foster, 1998). Another study using questionnaire survey found similar results (Coleman & McCalley, 1925). The

research participants were college students, including 54 present biters and 54 who had never bitten nails. All subjects were given the Bernreuter Inventory and a personal data sheet concerning childhood security feelings. The results

demonstrated that nail-biters rated significantly more introverted and neurotic on the Bernreuter Inventory and more current anxiety than non-nail-biters (Coleman & McCalley, 1925). Another study investigated nail biting and manifest anxiety in ten adult nail-biters and ten non-nail-biters (Klatte & Deardorff, 1981). According to a case report based on self-monitoring recordings by Mcclanahan (1995), anxiety was the most prevalent antecedent of nail biting. The results of these research studies are in accord with previous findings that nail-biters were more anxious than non- nail-biters.

The hypothesis that attributes nail biting to anxiety or nervousness was supported by the frequent association between anxiety and nail biting. However, this theory can be challenged by other findings that nail-biters do bite their nails in situations of both inactivity and anxiety (Hansen, Tishelman, Hawkins, & Doepke, 1990). The challenge against this theory was raised by researchers who proposed the arousal modulation theory which suggested that the function of nail biting was to calm them down in times of autonomic arousal (stress or anxiety) and to provide

stimulation (alert or stay awake) during periods of boredom or inactivity ( Hansen et al., 1990). Other researchers’ findings were consistent with the “arousal-modulation theory.” For example, in a 2007 study, the frequency of nail biting in 40

undergraduate nail biters at the University Psychology Department, United Kingdom were recorded in four settings: being left alone (boredom), solving maths problems (frustration, anxiety), being reprimanded for nail biting (contingent attention), and continuous conversation (non-contingent attention). The results proved that nail biting was evoked as a result of boredom and frustration or working on difficult tasks (Williams, Rose, & Chisholm, 2007). Therefore, it may be concluded that nail biting could be caused by other psychological states apart from “anxiety” or “nervousness” (Woods et al., 2001).

Other researchers found similar findings with Woods’ results (Deardoff et al., 1974). A study investigated the association between nail biting and manifest anxiety in children. The results indicated insignificant different manifest anxiety between nail- biters and non-nail-biters (Deardoff, et al., 1974). The findings of these studies in this context seemed to be more convincing. However, their research did not deal with and contribute much to the primary underlying cause of nail biting.

An interesting author postulates a paradoxical anxiety effect that nail biting is a sign of inhibited hostility or personal disarmament hence the nail biter may become nervous or anxious, as a consequence of nail biting (Ellerbroek, 1978). To date, no experiments had been done to test this hypothesis. This hypothesis seemed to be weak because many nail-biters did not experience anxiety while nail biting as demonstrated in the above studies. It is still inconclusive as to whether anxiety leads to nail biting or it may only be a co-occurring symptom with nail biting.

Other psychological issues are also reported by later researchers. For example, Michopoulos and his colleague (2012) reported on a case of a 66-year old man with severe finger mutilation as a result of serious nail biting for 6 years. His self-

mutilating behaviour was associated with severe diabetic neuropathy, impulsivity, and social isolation. Williams et al. (2007) carried out an intervention on 40

undergraduate student nail biters, using randomised allocation to four conditions of being left alone (boredom), solving maths problems (frustration), being

reprimanded for nail biting (contingent attention), and continuous conversation (noncontingent attention). Their findings revealed the most frequent occurrences of nail biting in the two conditions of boredom and frustration. This is evidence for the link between nail biting and particular emotional states.