The shape of the lower face is made up of several structures including your jaw bones, muscles and soft tissues like fat or skin. Whilst genetics play a huge part in terms of the shape and size of these facial structures, there are a number of other factors that come in to play. The main cause for lack of definition or sagging of the jawline is ageing and reduced face volume. One of the main reasons for developing a square jaw in both men and women can be overactivity of the chewing muscles which is usually caused by tooth grinding. Other factors that can give the appearance of loss of definition, jawline sagging or a square jaw is ageing and loss of volume in the cheeks or lack of chin prominence or neck sagging.