"Guillén de Castro’s The Force of Habit (La fuerza de la costumbre, c. 1610) is singular among comedias in that it takes the popular device of cross-dressed characters a step further, daring to ask whether gender is something that can be learned and unlearned, or if it is a fact of nature. The protagonists, a brother and sister separated at birth and raised apart, become the center of a discussion about nature versus nurture: Félix, brought up by his mother to speak softly, fear thunder and stitch with the women of the house, and Hipólita, raised with her father in a war zone to wield a sword like a soldier, horrify their parents and amuse onlookers with their complete reversal of feminine and masculine attributes. When the family is reunited, the father insists on making the siblings conform to traditional gender roles. While Félix teaches his sister how to wear high heels and Hipólita shows him how to use a weapon, the question of gender roles is complicated by the tangles of love. Castro thus uses the siblings to explore essential questions about the nature of identity and the limitations of a system in which the correct performance of gender is key to being accepted by family and friends alike"--