Definition: The tendency of people to develop preferences for people or things because of repeated exposure to them.
Reference: Robert Zajonc - nonsense words. Participants believed words that they were exposed to more often to have more positive meanings.
Insight: New things grab our attention, and we like things that are familiar. There is an optimal level of exposure to things for us to be both engaged and like the thing. This is what most product developers/artists are aiming for.
