Human groups are experts in developing and maintaining social norms. Many social norms have clear practical functions, such as regulating competition or facilitating coordination. Some other norms, however, have arbitrary functions and limited direct material consequences for the self or the group, but are nevertheless enforced. I define such norms as symbolic norms. Symbolic norms are prevalent across human societies. Given the discrepancy between the social importance and the functional opacity of these norms, it is important to understand how a seemingly neutral behavior can emerge as a symbolic norm and be adopted by the population. In this dissertation, I argue that a neutral behavior is more likely to evolve as a symbolic norm when it shows statistical correlation with a practical behavior on the population level. I call this the norm spillover effect. The norm spillover effect predicts that if, on the population level, followers of a practically beneficial norm happen to conduct a certain neutral behavior more often than practical norm violators, the social norm will spill over from the practical domain to the neutral domain. Thus, people will adopt and enforce that neutral behavior, and a symbolic norm will emerge. This dissertation uses agent-based models and an empirical experiment to test the norm spillover effect across two levels of analyses. First, agent-based models are used to test the evolutionary force behind the norm spillover effect on the population level. I argue that the statistical correlation between a practical and a neutral behavior creates an ecology that fosters symbolic norm following and enforcement. Second, an empirical experiment is conducted to examine the psychology of the norm spillover effect on the individual level. I argue that the perceived correlation between a practical and a neutral behavior increases the perceived direct function of and the pressure to conform to the symbolic norm.