Miller wrote:it is important to work on the causes of this habit on their minimization
i do not think that when a 3 year old child bite his nails you should put him on the couch, take the notepad and ask him "to tell you about his mother" . I was trying to make a joke about the classical picture of the psychoanalysis. When the kind is small, there are no that profound reasons for this habit. He just likes to chew things, no matter if that is a pen or his nails. there is no pen around, ok, the thumb will be ok then. I think that it is good to get rid of this habit when the kid is small, in other case it will be much harder. the young age is perfect for the forming of habits, both good and bad ones.