If a community widely believes that a certain behavior is acceptable, leading to an increase in that behavior, what underlying social mechanism is primarily at work?

The underlying social mechanism at work in this situation is the concept of social norms. Social norms are the unwritten rules about how people are expected to behave in a community. When many people in a community believe that a certain behavior is acceptable, it creates a shared understanding that makes others more likely to engage in that behavior as well. For example, if a community widely accepts casual littering as normal, more individuals may start littering because they see others doing it and feel it is acceptable. This shows how the beliefs and actions of a group can shape reality and influence individual behavior.