A longer lens can flatten and widen a face, whereas a wider lens can pinch/pull facial features into an ugly distortion. This is true because of the varying physical distance to your subject that accompanies your choice of lens. This is not just a consideration for portrait photography, but also comes into play when choosing a lens for filming actors. For the the full size images of the thumbnails above, see photographer Stephen Eastwood's site, or watch a video of how different distances (and accompanying lens choices) affect facial geometry by LensProToGo:

Link: Stephen Eastwood Lens Distortion Photo Strip

[via Gizmodo]