Verdict: changes the look, not the face. A hairstyle moves nothing under the skin. It changes where the eye lands, and for a face that is a few millimetres off on one side, that is the whole game. This is craft, not science, so no studies are needed. There is one measurable fact: you can check the result.
Start from the numbers, not the mirror
Most people style against a feeling: "my left side is worse." The feeling is usually right about which side and wrong about why. Before deciding anything, run the symmetry tool on a plain front photo. It gives eight left-right readings in millimetres: brow height, eye height, mouth corner, chin offset and the rest. Two of them decide most hair choices.
- Brow height difference. In 100 people measured before eyelid surgery, 93% had at least 1 mm of left-right difference in brow or lid position, and the average brow difference was about 1.8 mm. Under about 3.5 mm nobody else can see it. But it is the difference hair sits right next to, so it is the one hair can help most.
- Cheek and jaw fullness. The fuller side reads as the "bigger" side. Hair volume can balance it.
The part
A parting draws a vertical line that the eye reads as the centre of the face, whether or not it is. Use that.
Part on the side of the higher brow. The hair falling across the other side lowers the visual weight there, and the exposed side, with the higher brow, reads as open and lifted. Reverse it and the fringe covers the high brow while the low brow sits fully exposed under a wall of forehead, which exaggerates the difference.
A centre parting is the honest choice and the least forgiving one. It invites a straight comparison of left and right. If your chin offset or brow difference is under 2 mm, it is fine. If it is over 3 mm, an off-centre part is doing you a favour.
Move the part slowly. One centimetre changes which side gets the sweep. Take a photo at each position, in the same light, and run the tool on each. The readings do not change, because the face does not, but you can see which framing lets you stop noticing them.
Fringe
A fringe is the one tool that hides brow height difference completely. Any fringe that touches the brows removes the comparison. A curtain fringe, split off-centre, does the same while leaving the forehead partly open, and it lets you bias the split toward the higher brow so more of the low brow is covered.
If you do not want a fringe, a side-swept front section that crosses the forehead at a diagonal works the same way: a diagonal line makes it hard to compare two horizontal ones.
Volume
Volume goes opposite the fuller side of the face. If the right cheek or jaw is fuller, build volume on the left, at the crown and the side, so the silhouette as a whole reads balanced. Volume on the full side makes that side of the head bigger, and the whole face looks pulled that way.
The same rule covers a chin offset. If the chin sits to the left, a little more width on the right side of the hair counterweights it. You are balancing the outline, not the features, and the outline is what people see from across a room.
Length
Length works at the jaw. A cut that ends at the jawline draws a horizontal line right where the jaw angle is the most asymmetric point on a normal face, and it makes the comparison easy. Go either above the jaw, so the line sits at the cheekbone, or well below it, so the jaw is framed by hair and not by a cut edge. A slightly longer length on the side where the jaw sits higher tilts the frame back toward level.
For short cuts the same logic applies to the sides: a fade that is slightly higher on the fuller side and slightly lower on the slimmer side takes width off where there is too much and leaves it where there is too little.
Face shape decides the rest
Everything above is about left versus right. Which cuts suit you at all is about length versus width, and that is a measurement too. The face shape detector reads length-to-width and jaw-to-cheek ratios and gives the shape with the numbers behind it, so the advice about "round faces need height" or "oblong faces need width" starts from your actual ratio and not a guess.
The check
Two photos, same light, same distance, hair one way then the other. Run both through the symmetry tool. The readings will match, which is the point: the face did not change, and the version you prefer is the framing. Keep that one.
Sources
Brow and eyelid asymmetry in 100 patients: Macdonald KI et al., Journal of Otolaryngology-Head and Neck Surgery 2014;43:36. Perception thresholds: Wang TT et al., Aesthetic Surgery Journal 2017;37(4):375-385. 3D soft-tissue asymmetry in 48 adults: PMC3481973. Styling guidance is craft, not a study.