I will report on joint work with Peter Feller in which we show that a certain quantity associated to a ribbon knot K called the twisting number is bounded below by the doubly slice genus of K, which is the smallest genus of an unknotted surface in the 4-sphere whose intersection by the equatorial 3-sphere gives K. As applications, we answer a question of Tanaka by showing that the twisting number can be arbitrarily large, and determine previously unknown doubly slice genera for some knots in the knot tables.