The male jawbone is usually more heavily built throughout than the female one. It is squarer at the angles and the angles may also flare outwards. The chewing muscles (masseter muscles) also tend to be larger in men (sometimes a lot larger) and this adds to the width at the back. Many people expect mens jaws to be wider in relation to their cheeks than women's but in fact, the width of the jaw relative to the width of the cheeks tends, on average, to be almost exactly the same for both sexes.
I don't believe that the jaw is as important a gender sign as many people think. For archaeologists it is very important and is one of the clear distinctions between male and female skulls but when we are looking at someone's face, our attention is not focused much on the jaw and features like square jaw angles are not only hidden around the side of the face but are also hidden a lot of the time under a fairly thick layer of soft tissues. Another thing to bear in mind is that a relatively wide jaw is often seen on beautiful faces like those of Gwyneth Paltrow, Penelope Cruz and Angelina Jolie:
Picture courtesy of Remy Steinegger
The width of the jaw and the height of the chin are closely linked and you should not alter one without considering how it will affect the look of the other. Basically, if you shorten the chin you will make the jaw look wider and conversely, if you narrow the jaw you will make the chin look taller.
Important Note on Hormones:
Estrogen can make your jaw narrower. It doesn't affect the jawbone but it does reduce muscle mass and when this affects the chewing muscles, it can make the jaw seem a little narrower. Your surgeon should be aware of this and should take your hormonal status in account when planning jaw surgery. I suggest waiting until you have been on a full dose of estrogen for at least a year before you consider jaw surgery unless it is clear that a reduction of the chewing muscles will not be enough to narrow the jaw the necessary amount in your case. You can have botulinum toxin (Botox) injected into the chewing muscles to reduce them (see below).
The jaw corners can be reduced by cutting down the bone and this can extend along the jaw to narrow and reduce the whole jawline. The incision is usually made through the mouth down in the gum line so the scars are not visible. To make the jaw even narrower the large chewing muscles (masseter muscles) that attach to the corners can be surgically reduced but this is quite traumatic, causing pain and can take a while to recover from. Botulinum toxin is now commonly used to achieve the same result (see below).
The surgeon needs to be careful to avoid the “inferior alveolar nerve” which runs along inside the jaw. This nerve supplies sensation to the chin, lower incisors and bottom lip so these areas can become numb if it is damaged during surgery. Here's a diagram:
Facelifts After Jaw Surgery:
On older FFS patients chin and jaw surgery can leave the soft tissues around the lower face and neck more slack than they were before. This happens partly because the bones have been reduced but also because chin and jaw surgery can cause quite a lot of swelling and that swelling stretches the soft tissues. Younger patients don't have this problem as the soft tissues are very elastic and will tighten back up around the new jaw and chin as the swelling goes down. The age at which you might need to consider a facelift after chin and jaw surgery will vary from person – one 50 year old might have very toned soft tissues while another 40 year old might already be having noticeable sagging. You would normally have to wait a few months after jaw and chin surgery before having a face lift.
As I mentioned above, the botulinum toxin (widely known as Botox) can be injected into the chewing muscles. It does not affect their function and you'll still be able to chew as normal but it does cause the muscles to shrink somewhat. It is not yet known whether this can provide a permanent solution or whether you would need to top up every few months. Here's a link to the Wikipedia page on botulinum toxin: