A new study shows that cells lurking deep in the mouse hypothalamus help determine whether it fights or mates. The study, published February 9 in Nature, shows that when these neurons are quieted, mice ignore intruding males they would otherwise attack. Yet when the cells are activated, mice assault inanimate objects, and even females they ought to court. The cells lie within an area of the hypothalamus with known links to violent behavior. An electrical jolt to this vicinity causes cats and rats to turn violent, but neurophysiological experiments conducted decades ago stimulated too big an area to identify the specific brain circuits, let alone the individual neurons, involved in aggression.