High moisture content does; so the recipe, baking time, and temperature must be adjusted to retain moisture. Binding the water in butter, eggs, and brown sugar (it contains molasses, which is 10 percent water) with flour slows its evaporation. The dough needs a little extra flour, which makes it stiffer. The stiff dough spreads less, less liquid evaporates, and the cookies are thicker.
Mass also helps cookies stay moist--big dollops of dough make softer and chewier cookies than tiny spoonfuls of dough. Bake these thick cookies for a shorter time at a high temperature to firm them quickly and minimize spreading. Most important, don't bake them too long--remove from the oven when the cookie rim is brown and at least 1/3 of the center top remains pale. The cooked centers will be soft.