Reactance

Reactance is the resistance people show the moment they sense a choice is being taken from them, and it is why a reward or a nudge that feels imposed can produce less compliance rather than more. Nir Eyal's design rule follows from it: do not force the new behaviour, present an implicit choice between the old familiar way and a new, more convenient one. The mechanism is plausible and widely observed. The headline effect sizes attached to reactance research have not held up.
What it is
Most models of influence treat resistance as friction: the request was not attractive enough, or the incentive was too small, and the fix is more of whatever was offered. Reactance describes a different failure, one where the offer itself is what produced the refusal. The trigger is not the content of the request but the sense that a choice is being removed, and the response is fast, often out of proportion, and frequently aimed at the person doing the asking rather than at the thing being asked for.
The practical consequence is that compliance is not a monotonic function of pressure. Past a certain point, pushing harder produces less. A reward attached to a behaviour a person was already inclined towards can make them less inclined, because the reward reframes a choice as an instruction. A rule imposed for someone's own good invites its own violation, not because the rule is wrong but because obeying it now means accepting that someone else sets the terms.
The theoretical home for this is autonomy. In self-determination theory, Deci and Ryan's account of motivation, autonomy is one of three basic psychological needs alongside competence and relatedness. Reactance is what fires when the first of those is violated. That framing is useful because it explains why the effect is not merely stubbornness: what is being defended is the same thing that sustains motivation in the first place, which is why an imposed version of a behaviour can be less durable than no version at all.
Nir Eyal's contribution, in Hooked, is to turn this into a rule a product team can apply. His formulation: "Companies that successfully change behaviors present users with an implicit choice between their old way and a new, more convenient way." The word doing the work is implicit. The user is not asked to agree to a change, told they should change, or rewarded for changing. They are shown two routes and one of them happens to be easier. The decision stays theirs, which is precisely what stops the resistance from firing.
This also marks a boundary on the rest of the persuasion literature. In the foot-in-the-door research, a small compliance makes a larger later request more likely, but only within limits. Burger's 1999 review of that literature identifies reactance as one of the moderators: if the second request is large enough to feel like an imposition, the technique reverses. Reactance is the design failure mode that persuasive design as a discipline has to route around, and it can work directly against other levers. Social proof pulls a user towards a behaviour by showing them that others do it, and reactance pushes back the moment the same signal reads as pressure rather than information.