What makes cookies chewy vs. fluffy?

er rather than shortening. I saw this once on one of Alton Brown's episodes a few years ago on Food Network. Maybe true because I always use er, even if recipe calls for shortening and have never had a fluffy, puffy cookie. Mine are always nice and chewy, just like I like them. This year for Christmas I made ginger cookies, oatmeal scotchies, peanut er and potato chip cookies. All were nice and chewy.