Reciprocity
People feel the need to reciprocate when they receive something
Definition
Reciprocity is a social norm of responding to a positive action with another positive action, rewarding kind actions. In the context of digital product experiences, users are more likely to engage with your product if you first provide them value. They'll be more likely to trust you and reciprocate. That's even more important when you're about to ask for something big from your users (sign up, paywall, etc).