The myth in our neck of the woods goes that it makes the lip bulge out and reduces chin size.
I don't know how true it is, but the only time our 3 month old doesn't bite his lip is when he's chewing his fingers. We accept that as the lesser evil. He doesn't have a pacifier yet.
Any tips?
Update
He gave up this behaviour by 6 months or so.
I know for a fact that sucking the lower lip does cause a bulging misshaped blob on the lower lip. My son fell at about 18 months cutting his lower lip. He sucked the scar for the next 3 years creating a large bulging lip that his dentist said would likely not reabsorb. I taught him to place his tongue between his teeth and created a tongue sucker instead (not a good idea). The lip did normalize after a few months (but the tongue sucking has persisted into adulthood).
Lip biting in an infant would likely be related to the position of his upper and lower dental arch. If the lower jaw is significantly smaller than the upper there is an anatomical positioning of the upper arch over the lower lip that looks like biting. This would be genetic and could only be addressed when he is much older.
In this situation, the "biting" does not reduce the chin size, but the smaller chin size is the reason for the "biting.
Placing fingers and other objects in the mouth would keep the upper and lower gums separated and prevent the "biting" of the lower lip, but will not actually change the position of the dental arches.
As a child grows, sometimes a smaller upper or lower grows and becomes more proportional with the other.
Older children with weak oral muscles sometimes stabilize their muscles by biting the lower lip, jutting the jaw forward, or torquing the jaw to one side. A 3 month old is far to young for this to be the situation.
I can't scientifically debunk the myth you mention, but I've never heard of it and I don't think it's true. Keep in mind that the most sensitive part of an infant's skin is the mouth. Later on the fingers develop sensitivity too, but this early the mouth is the primary stimulation point a baby has.
if he's not using a pacifier (and assuming you feel that a pacifier is not worse than biting the lip) then try to introduce a pacifier and see what happens.
You could also offer various teething toys, even if he's not actually teething yet. This assumes that he has developed enough motor control to hold a toy and move it where he wants it.
