Motivation and habit
Reciprocity
Also called Reciprocity principle
People feel they should give back when they've been given something, so offering value first makes them more willing to give in return.
Give someone something useful and they feel a small pull to return the favour. It's one of the most widespread social norms there is.
In a product, give before you ask. Let people get value before asking for an email address, a review or a payment, and the ask feels fair instead of like a toll.
In practice
A design tool that lets people make and download a finished image before asking them to sign up tends to get more sign-ups than one with a registration wall at the door.
Common mistake
Gifts with strings people didn't agree to, like a free download that silently signs them up for a newsletter. It turns reciprocity into a trap.
Where it comes from
Sociologist Alvin Gouldner described the norm of reciprocity in 1960. Robert Cialdini made it one of six principles of influence in Influence (1984).