Thank you for your question. I must admit that I was initially unfamiliar with the term "mewing" and had to do a bit of research into what this meant.
Mewing is a concept developed by Drs Mike Mew and John Mew that encourages a proper way of positioning the tongue and jaw muscles so as to influence the way the face grows and also purports to have a positive impact on an adult's facial structure.
Although this termed "Mewing" is a new term, the concept behind form follows function is definitely not new.
Melvin Moss in 1962 introduced the concept of functional matrix hypothesis which is a theory that suggests that the growth of the face occurs as a response to functional needs and mediated by the soft tissues in which it is embedded.
Some of the possible functional influence on facial growth include the resting tongue posture and chronic mouth breathing as a result of nasal obstruction.
A good example of the result of chronic mouth breathing due to nasal obstruction is the "Adenoid facies" or long face syndrome, a termed coined by C.V. Tomes in 1872.
When one has a chronic nasal obstruction, the neck extends upwards to improve the airway, there is downward and backward rotation of the lower jaw and the tongue assumes a more forward position, all of which contributes to a longer face growth.
Though there is an association between facial growth and functional influence (e.g tongue posture, mouth breathing and muscles), the relationship is an association.
A closer look at the evidence suggest that the relationship between nasal obstruction and a long face syndrome is not causal. This means that not all children with nasal obstruction grow to have a long face while a large proportion of children with chronic nasal obstruction are observed to have a long face.
This I believe is because growth of the face is also influenced by other factors including genes , growth pattern and the severity of the disordered breathing.
There is currently no published evidence on the effect of mewing, though there are certain ideas in mewing (e.g proper tongue posture is important in the stability of anterior open bite correction) that are well accepted by the orthodontic community.
However, there is little in the concept that is supported by proper research and data.
