Betty sent christmas cards to everyone in her address book and received christmas cards in return from most of the recipients. this result is consistent with the social behavior known as

This result is consistent with the social behavior known as "reciprocity". 

Reciprocity in social psychology refers to a social standard of reacting to a positive activity with another positive activity, remunerating kind activities. As a social build, correspondence implies that in light of cordial activities, individuals are as often as possible more pleasant and considerably more helpful than anticipated by the self-interest model. 

Pilgrims is the answer