The first mention of making criss cross fork marks in Peanut Butter Cookies was in a Pillsbury cookbook from 1936.  It didn’t specifically mention why one should make the criss cross fork pattern, but it’s widely assumed that it was simply to flatten the dough down.    The fork marks have now become a hallmark of these cookies.  With the dough being quite denser than other cookie doughs, it does also help make sure that the cookies bake evenly.