  Social behaviors are often contagious, spreading through a population as
individuals imitate the decisions and choices of others. A variety of global
phenomena, from innovation adoption to the emergence of social norms and
political movements, arise as a result of people following a simple local rule,
such as copy what others are doing. However, individuals often lack global
knowledge of the behaviors of others and must estimate them from the
observations of their friends' behaviors. In some cases, the structure of the
underlying social network can dramatically skew an individual's local
observations, making a behavior appear far more common locally than it is
globally. We trace the origins of this phenomenon, which we call "the majority
illusion," to the friendship paradox in social networks. As a result of this
paradox, a behavior that is globally rare may be systematically overrepresented
in the local neighborhoods of many people, i.e., among their friends. Thus, the
"majority illusion" may facilitate the spread of social contagions in networks
and also explain why systematic biases in social perceptions, for example, of
risky behavior, arise. Using synthetic and real-world networks, we explore how
the "majority illusion" depends on network structure and develop a statistical
model to calculate its magnitude in a network.
