Motivation and habit
Commitment and consistency
Also called Foot in the door
Once people take a small step or state a position, they tend to act in line with it, so small commitments make bigger ones likelier.
People like to see themselves as consistent. After agreeing to something small, especially in public or in writing, they're more likely to agree to something larger that fits it.
Products use this in onboarding: ask for one small, easy step first, such as picking a goal, then build on it. A choice people made themselves feels like theirs to follow through on.
In practice
A fitness app that asks people to choose their own weekly goal on day one tends to see them keep it more often than if it assigns one.
Common mistake
Stringing people along with escalating asks they'd never have agreed to up front. It's a classic manipulation technique, and people recognise it.
Where it comes from
Jonathan Freedman and Scott Fraser demonstrated the foot-in-the-door effect in 1966. Robert Cialdini made commitment and consistency one of the principles in Influence (1984).