Some fish are famously able to develop into either sex, depending on social circumstances. One study of a coral reef fish, the goby Gobiodon erythrospilus, placed juvenile fish with either an adult male or an adult female (Hobbs et al. 2004).

Of the 12 juveniles placed with a male, 11 became female. Of the 10 juveniles placed with an adult female, 6 became male. What method can we use to test whether the social context of the juvenile fish affects which sex they become?