The tongue is responsible for applying an outward force to the teeth and jaws, which helps to spread the teeth out and also help the upper jaw grow in size. The lips and cheeks apply an inward force, restricting the teeth from being too flared out and also help to restrict the growth of the jaws.
When the outward forces of the tongue, and the inward forward of the lips and cheeks are balanced, the jaws develop normally and the teeth fit in to the jaws with little or no crowding. However, there are several habits that can affect this balance of soft tissue forces.
For example, children who breathe through their mouth all the time will have a tongue that does not rest on the roof of the mouth. This removes the outward force against the upper jaws and teeth, and makes the upper jaws too narrow and upper teeth crowded. Children who have overactive lips when they swallow increase the inward forward against the lower jaws and teeth, which restricts the growth of the lower jaw and makes the lower teeth crowded.
Using a myofunctional trainer together with practicing certain exercises at an early age can help to correct these habits, and help the jaws and teeth develop normally. This training can take up to 2 years altogether, and can remove the need for braces in the future.