It is true, you would never suspect facial symmetry is a thing, and even though the ones of Sir Paul (McCartney) are close to symmetrical, they're not perfect, take the image below. Please note. This is the 'cheating way'

Now according to psychology (I'm not a psychologist, it's just what I've been told!) your brain should prefer the image on the right because it's perfectly symmetrical.

Now no face is going to be perfectly symmetrical, so how was this done?

Part camera, mostly editing.

The left image is the original unedited image (if it wasn't obvious), so how do you go about making it look symmetrical?

First of, and this is the really hard part you need to take the picture perfectly, so dead on centre, no head tilt to the side, the light needs to be perfectly even as well or otherwise the brain can pull up on the subtle differences that the image has been altered.

Secondly, (crop and rulers/guides in photoshop are a big help here), you need to cut the image in half and mirror the side so the picture is a mirror image of itself down the nose. At this point you will find that one side of the face is better. (It can get complicated as you get things correct due to any irregularities if the photo wasn't done correctly).

Thirdly, the image will look weird as a mirror image of it self, so you have to bring things like the hair and body parts BACK from the original half of the image.

A litle tip if you fancy trying this yourself, layer masks are you friend.