The pitch is simple: chew something hard, the jaw muscle grows, the jawline sharpens. It is intuitive, it sells a lot of gum, and it has now been tested in a randomised controlled trial. The mirror looked the same.
The trial
Fifty-eight adults were randomised at Yonsei University. The training group chewed gum three times a day for six months. The control group did nothing. Maximum bite force rose significantly in the training group by month three. But when the researchers looked at why, the increase came from a larger occlusal contact area, the teeth meeting better, not from a bigger muscle. The paper's conclusion is direct: gum chewing training had no effect on masseter thickness or mandibular morphology.
Six months, every day, no change in the muscle you can see and none in the bone.
The "ten times harder" gum
Mastic gum is marketed as roughly ten times harder than regular gum. That figure exists only on seller blogs. There is no texture analyser measurement, no pressure figure, nothing in peer-reviewed literature. What mastic actually has evidence for is antibacterial activity and plaque reduction, which is a fine reason to chew it and not the reason it is sold.
The chewing-side story is messier than TikTok says
You have probably heard that chewing on one side makes that side of the face bigger. The honest version has two data sets that disagree. A twin study using 3D scans found chin volume larger on the side opposite the habitual chewing side, the reverse of intuition. The largest cohort, 748 adults from the Northern Finland 1986 birth cohort, found no statistically significant effect of chewing preference on facial asymmetry at all. What does hold up: the masseter is about 1 mm thicker on the preferred chewing side. One millimetre, on a muscle nobody can see through the cheek.
What a jawline responds to
Bone is fixed. The masseter barely moves. What changes how a jawline reads is the layer in front of both: fat and fluid under the chin and along the edge, and the head posture that either shows the mandible or buries it. Lose fluid overnight and the edge sharpens by morning; sit with your head forward and it disappears by noon.
That is why Symmi's jaw width reading is one of the few structural readings tagged trainable: what the camera measures at the jaw angle is bone plus soft tissue, and the soft tissue part moves. The plan for it is not gum. It is drainage, gua sha along the edge, a two-minute cold pass for the fluid, and the chin-tuck and wall-posture drills that stop the head from creeping forward. Then a rescan on Day 7 to see whether the number moved.
Sources
Jung J.H. et al., Journal of Oral Rehabilitation 2024;51(12), doi:10.1111/joor.13830. Heikkinen E.V. et al., Acta Odontologica Scandinavica 2022;80(3):197-202. Northern Finland Birth Cohort 1986, Acta Odontologica Scandinavica 2024.