Watching someone pick up a carrot instead of a cookie makes other people more likely to pick up the carrot, too. On the other hand, watching someone give in and eat the cookie makes people less likely to resist the sweet temptation of the dessert–even if those people have no other social interaction.
People tend to see self-control as a personality trait, not as something influenced by those around them. But according to research highlighted by Jonah Lehrer, self-control and the ability to resist temptation both have a strong social component. That may be why fast-food restaurants bombard the airwaves with images of people giving into temptation–to overwhelm other people’s ability to resist. The research also suggests that self-control can be learned. If just one person starts making the decision to eat healthier, everyone around them may be more likely to eat healthy, too.
Source: The Frontal Cotex