The way you light a face has a big impact on how we see it.

Narrow lighting (an example is shown below) is achieved by photographing the side of the face away from the main light. In this case the light is coming from an open doorway; the background appears almost black because the room behind is much darker than our model's face. 

An example of narrow lighting

Narrow lighting makes the face appear slimmer as the eye sees what is lit; shadow can be a good friend! 

With broad lighting the face is photographed on the side of the face that is lit; an example is shown below:
Broad lighting a face

Martha looks gorgeous in both pictures, but you can see how the face appears broader when it's fully lit. 

Children generally look great with broad lighting, but more often than not most adults will appreciate having narrow lighting on their faces. The same applies to a body - if it's lit then the eye sees it, so if you want to fool the eye for a flattering image then use narrow lighting. 

I teach this and many other aspects of portrait photography on my Portrait Training day courses at Lacock Photography.