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Parents often raise concerns about rhythmic and repetitive behaviors such as rocking, head banging, and thumb sucking. Some are common; others are unusual and idiosyncratic. The high prevalence and heterogeneity of repetitive behaviors raise questions about the origins and biological significance of these common pediatric concerns. Learning theorists contend that rhythmic movements begin as normal behaviors that are reinforced over time. If these activities are pleasurable or tension relieving, they tend to be repeated. However, prenatal ultrasounds show the human fetus sucking its thumb by the 28th week of gestation, suggesting that some complex repetitive movements represent genetically coded, species-specific behavior. Observations of picking and grooming behaviors among primates suggest that hair twisting and pulling may originate from social behavior of our human ancestors.

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The brain is a rhythmic organ. The brainstem, hypothalamus, and other mesencephalic regions pulse and cycle, suggesting that some repetitive behaviors arise from normal neuronal regulation of the primitive brain. The emergence of abnormal repetitive behaviors after severe brain injury and the frequent stereotypies in individuals with severe cognitive disabilities indicate that human cortical functions may inhibit “deeper” and more primitive repetitive behaviors.

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Regardless of their origins, repetitive behaviors are mediated by neurotransmitters. Observations of rhythmic movement disorders in cases of human amphetamine overdose and following intracerebral injection of dopamine and amphetamines in animal studies have implicated dopamine (which mediates behavioral reward systems), serotonin, gamma-aminobutyric acid (GABA), and endorphins.

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The clinical significance of repetitive behaviors in children depends on the characteristics of the behavior (the kind of movements, their frequency, and timing), the context (the age of the child, when it occurs, what makes it worse), and the impact of the behavior on health and function. For example, thumb sucking occurs commonly enough in children of age 3 or 4 years to be considered normal, but it may cause an orthodontic problem if it occurs very frequently. It is normal for a school-aged child to suck his thumb briefly as he falls asleep, but habitual thumb sucking in the classroom may indicate a psychological problem related to anxiety or inhibition and may interfere with social function.

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A consideration of the function and meaning of the behavior helps us in classifying repetitive behaviors as habits, stereotypies, tics, or compulsions.

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Habits

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Habits are assembled routines that link sensory inputs with motor outputs through procedural learning. Common habits, including rocking, head rolling, head banging, hair twirling, thumb sucking, nail biting, nose touching, teeth grinding, and joint popping, are not strongly associated with neurodevelopmental disorders or neurological disability.

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Most habits serve a self-soothing function at times of stress or boredom. Children engage in these behaviors voluntarily but may learn to inhibit the behaviors through social reinforcement (pleasing parents or avoiding humiliation in school). Indeed, some habits may develop through imitation and social reinforcement: Children may learn to pop their knuckles from other ...

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