Konrad Lorenz stated in his 1963 classic, "On Aggression," that human behavior is shaped by four main, survival-seeking animal drives. Taken together, these drives—hunger, fear, reproduction, and aggression—achieve natural selection. E. O. Wilson elaborated in "On Human Nature" that aggression is, typically, a means of gaining control over resources. Aggression is, thus, aggravated during times when high population densities generate resource shortages. According to Richard Leakey and his colleagues, aggression in humans has also increased by becoming more interested in ownership and by defending his or her property. However, UNESCO adopted the Seville Statement of Violence in 1989 that refuted claims, by evolutionary scientists, that genetics by itself was the sole cause of aggression.