High activity in a relatively poorly studied group of brain cells may be related to aggressive behavior in mice, according to a new study from the Karolinska Institute in Sweden. Using optogenetic techniques, scientists were able to control aggression in rodents by stimulating or inhibiting these cells, as reported in Nature Neuroscience. The findings of this work contribute to a better understanding of the biological mechanisms behind aggressive behavior. Hostility is a behavior that is found throughout the animal kingdom and that shapes human life from the first encounters in the school yard to, in its most extreme expression, the armed and global conflict. Like all behavior, aggression originates in the brain, but the identity of the neurons that are involved, and how their properties contribute to the stereotyped expression that often manifests itself in interpersonal conflicts, remains largely a mystery. Now, researchers at the Karolinska Institute now show that a previously unknown group of neurons in the ventral premamillary nucleus (PMv) of the hypothalamus, an evolutionarily well-conserved part of the brain that controls many of our fundamental impulses, plays a key role in the initiation and the organization of aggressive behavior.