Occassionally, Gerald and Elsa like to "spice up" their lovemaking and engage in mutually gratifying sex play in which one of them uses a feather brush to strike the other. They feel it is more the sense of dominating or being dominated that each of them finds gratifying, and they frequently switch roles so that one time Gerald uses the brush on Elsa, and the next time she uses the brush on Gerald. Their behavior is typical of __.