Two powerful sources of influence are groups and interpersonal influence. Groups can influence through ingroup bias which is shown in The Simpsons when Springfield residents are initially skeptical of Lyle Lanley but join his monorail project once others do. People can also be led to conformity, as when the residents board the unsafe monorail due to social pressure. The six categories of power are reciprocity, social validation, authority, scarcity, commitment and consistency, and leverage. These techniques can be used for manipulation, such as using friendship or creating artificial scarcity to influence decisions. The Movementarians in "The Joy of Sect" effectively apply these influence techniques to grow their cult.