My two nieces are 4 and 6 and they both speak with a lisp, but so does their mom. Is this a learned behavior like accents or is it hereditary?
Could be a combination of both, depending on the reason. If there is some genetic physical anomoly that causes the mother to lisp, this could be inherited by her children. But children also copy whoever (and whatever) they hear, as every parent who’s uttered a curse word or vulgarity in front of their youngsters can testify.
my son has one, but he has both a very narrow mouth and a relatively long tongue. So I’m guessing it’s at least (in his case) partially of physical origin.
Another contributing cause could be lost teeth. At that age my son had a slight lisp due to never knowing exactly how many front teeth he was going to be working with for the next few months.