Humans tend to return good deeds: use this social psychology law in user interface design to gain users’ trust and motivate engagement with your site or app.

Reciprocity (social psychology)Wikipedia
The Reciprocity Principle: Give Before You Take in Web DesignNielsen Norman GroupFebruary 16th, 2014
Applying Behavioral Principles to Personalize Content Using Social MediaUX MattersAugust 19th, 2013
Robert Cialdini on the importance of reciprocity (video)YouTubeJuly 8th, 2009