Nail biting

Most of us know at least one person who bites their fingernails or used to as a child. Nail-biting is an occasional behaviour for some people but also a habit others struggle to stop. Overall it's estimated that 20-30% of the general population habitually bite their nails, with children making up the majority of this group [1] Jump to reference section: [1] . The habit commonly begins in middle childhood at 20-33% of the population and peaks in adolescence at 45% of this population [2] Jump to reference section: [2] .

Beyond the loss of cosmetic appeal, nail-biting can lead to infections and psychological unrest. Clinically, habitual nail-biting is called onychophagia. It falls among other pathological grooming behaviours like skin picking and thumb sucking. While the heritability of onychophagia has been established, studies on specific genes associated with it are still ongoing. The lack of clarity is partly due to the varying experience of nail biting among individuals. Regardless, there is significant progress in understanding the genetic basis of nail biting:

>30%: Percentage of nail-biters with nail biting family members.

50%: The increased likelihood of identical twins sharing nail biting tendencies compared to fraternal twins.
3-4 fold: The increased likelihood of developing nail biting for children whose parents both have a history of nail biting [3] Jump to reference section: [3] .

The genetics of nail biting

Pathological grooming behaviours are controlled by a group of genetic factors, including those that determine obsessive-compulsive disorders (OCD) [4] Jump to reference section: [4] . Due to the thin line between these two, OCD patients may sometimes show grooming behaviour such as nail biting. However, individuals who bite their nails do not necessarily show OCD symptoms. 

Nail biting has been specifically associated with the DLGAP3 gene, also known as the SAPAP3 gene [5] Jump to reference section: [5] . This gene contributes to the formation of synapses. A synapse is a molecular junction where impulses are passed from one nerve cell to another. Therefore, DLGAP3 activity affects the intensity of sensations related to chronic grooming behaviour like nail biting. Two particular variants of the gene are associated with onychophagia; the A variant of rs4653109 and the G variant of rs6662980 [6] Jump to reference section: [6] .

Variant rs4653109 is located at the far end of the SAPAP3 gene. This location means that it determines the attachment or detachment of specific molecules for physiological function. Specifically, the variant initiates an interaction between molecules that are responsible for transmitting sensations through the nervous system. The presence of the A variant may cause an imbalance in these interactions leading to nail biting urges. As evidence for the existence and role of this variant, research has demonstrated that removing sections from the SAPAP3 gene causes anxiety and excessive grooming behaviour in mice models [7] Jump to reference section: [7] . Additionally, drugs used to treat nail biting aim to reduce the imbalance in the nervous system and restore normal transmission of sensations.

Variant rs6662980 is located within a region of the gene that is not directly involved in the formation and interactions of molecules. However, its presence affects the stability and rate of the process. The presence or absence of the G variant affects the stability and rate at which these molecular interactions occur. Specifically, it may cause an imbalance in the sensations processed by the brain and how they relate to behaviour such as nail biting.

Other factors causing nail biting

People can habitually bite their nails for reasons beyond the highlighted heritability factors. Some people bite their nails when they are nervous to help them relieve stress and anxiety. Others view nail biting as a habit they can use to distract themselves from frustration, boredom, and long waiting times. Similarly, when individuals are deeply concentrating on a subject or thinking hard about a task, they may bite their nails unconsciously [8] Jump to reference section: [8] . 

On the other hand, psychological and emotional disorders may also cause pathological nail biting. Nail biting has been reported as one of the symptoms in individuals with mental health conditions, including obsessive-compulsive disorder (OCD), major depressive disorder(MDD), attention deficit hyperactivity disorder (ADHD), and Tourette syndrome. 

Risk factors may also increase the likelihood of nail biting tendencies, especially in children and teenagers. Practices that affect feeding habits in children such as the breastfeeding approach, premature and prolonged feeding using bottles, and extensive use of pacifiers and dummies may cause nail-biting in children as they grow. More risk factors include low and high birth weight and conflict within the family that may affect self-esteem or cause anxiety. 

What can you do about it?

Potentially, continued nail biting may cause damage and infect your nail bed or aid in the transmission of pathogens that cause oral infections. If you find yourself struggling with nail biting, first you should seek to avoid triggers such as stress and anxiety. If you are dealing with children and teenagers with habitual nail biting, it is important to create a supportive environment with confidence-boosting activities. Some people also find it useful to apply a distasteful/bitter nail-coating or make their nails softer using olive oil to help prevent themselves from biting their nails. 

Alternatively, habit reversal therapies can also be useful. Examples include chewing gum in place of biting nails and regular manicure and nail hygiene to make unbitten nails more appealing. Similarly, cognitive behavioural therapy (CBT) can help create behaviours for coping with nail biting urges. For instance, you can set targets and reward yourself for not chewing your nails. Regardless of the approach used, identifying your nail biting triggers is key to coming up with appropriate solutions.