Sexually dimorphic instinctual behaviors that set females and males apart are found across animal clades. Recent studies in a variety of animal systems have provided deep insights into the neural circuits that guide sexually dimorphic behaviors, such as mating practices and social responses, and how sex differences in these circuits develop. Here, we discuss the neural circuits of several sexually dimorphic instinctual behaviors in rodents, flies, and worms-from mate attraction and aggression to pain perception and empathy. We highlight several salient similarities and differences between these circuits and reveal general principles that underlie the function and development of neural circuits for dimorphic behaviors.