"Researchers had adults, four-year-olds, and two-year-olds say “bed” repeatedly. But scientists filtered the sound so that the subjects heard themselves through headphones pronouncing it as “bad.” Adults spontaneously compensated, and changed so that the word sounded correct to their own ears. They wound up saying “bid.”So, how do young children learn to speak correctly if they can't even hear themselves say something incorrectly?
Four-year olds also adjusted their speech.
But the two-year olds kept saying “bed” even though they kept hearing “bad.”"
The answer to this question has great import, especially to speech therapists whose job is to "fix" this problem. ENTs are often consulted as well to ensure a child does not have a tongue tie or hearing loss that can also lead to speech difficulties in a young child.
It's possible that young children depend on their parents to correct them when they say something incorrectly.
However, nobody quite knows for sure.
A follow-up study (not sure if will pass muster with an IRB) would be to have 2 sets of 2 year olds... one group where the parent constantly corrects any speech errors the child makes immediately. The other set would have parents NEVER correct a child's speech errors and let the child learn on their own.
Two possible outcomes... By the age of 5 years, there may be no difference in speech skills between the two groups OR there is a big difference and all the parental anxiety over their 3 years old child's inability to say "pizza" may be unnecessary (along with speech therapy sessions)... after all, they will "grow" or "learn" out of it with time.
Toddlers Don't Monitor Their Own Speech. Scientific American 12/24/11
Children's Development of Self-Regulation in Speech Production. Current Biology, 10.1016/j.cub.2011.11.052