infants, thumb-sucking is a normal phenomenon. Mostly
when the baby has not had his fill of sucking the breast
or the bottle, it is very natural for him to start sucking
his thumb. It is only when the baby grows, that the habit
becomes a worry in the minds of the parents.
Also, thumb sucking comforts the baby in the absence of the
mother, by providing comfort that is within the reach of the
baby. As you leave the baby, he sucks harder. He sucks instead
of crying; uses the energy that would have gone into crying. It
is so basic that your child may combine it with other forms of
comfort, like cuddly toys, etc.
In some children, thumb sucking may also be a result of
emotional insecurity. When parents are not able to give
sufficient time and attention to their child, the child looks
for security in other places.