Throughout this chapter we have discussed how people interact and influence one another’s thoughts, feelings, and behaviors in both positive and negative ways. People can work together to achieve great things, such as helping each other in emergencies: recall the heroism displayed during the 9/11 terrorist attacks. People also can do great harm to one another, such as conforming to group norms that are immoral and obeying authority to the point of murder: consider the mass conformity of Nazis during WWII. In this section we will discuss a negative side of human behavior—aggression.