Aggression is a normal innate behavior utilized for access to food, territory and mates by essentially all species of animals, including humans. Levels of display of aggression vary widely among individuals, however, and it generally is not known how much of this heterogeneity is genetic and how much is socially induced. Probably both mechanisms influence the expression of the behavior in all organisms, and the proportions of each that are involved vary widely between individuals. Unbridled aggression, in the form of violence, is a peculiarly human manifestation of this behavior, and when one adds the use of weapons capable of inflicting deadly damage to individuals and masses of individuals, it is a serious problem in society. Indeed weapons allow the least fit of individuals to become dominant protagonists in our society. In animal species, conspecifics sometimes kill opponents as well, but more commonly members of the same species engage in ritualistic stepwise-increasing-intensity-displays of fighting abilities. Winning and losing decisions can be made anywhere along the steps of such an intensity ladder. The roots of aggression are biological but there is little concrete information of how and where in the nervous system the seeds of violence are sown. In this application we propose to use a Drosophila model of aggression that we pioneered the use of in its modern form. Of all the available models for aggression, the Drosophila system offers the greatest ease and reproducibility of genetic manipulation within the nervous system down to single neuron levels. These manipulations can readily be combined with quantifiable behavioral measures in attempts to understand this complex behavior. Recently, using a novel strategy called intersectional genetics, we identified and manipulated in behaving animals single serotonin, dopamine and octopamine (fly equivalent of norepinephrine) neurons that all are involved in aggression. Thus, a single pair of serotonergic neurons found via this route, facilitat going to higher levels of aggression during fights, while a single pair of dopaminergic neurons are required to generate short term winner effects. In this application we ask a series of questions about the high-level aggression used by males to win fights. (1) What neurons and circuits are involved in going to high-intensity levels during fights, how do they work and how do they form during development? (2) What genetic or wiring differences exist in the nervous systems of the parent strain of flies and a hyper-aggressive line we generated called bullies that fight at higher intensity levels and always win fights against the parent strain? (3) Can we explain at cellular and circuit levels the learning and memory that takes place during fruit fly fights and accompanies the generation of winner and loser flies with changed aggression profiles? This application addresses the question of whether science and the study of model organisms can explain even a small part of the serious and pressing issues surrounding the root causes of human violence.