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