opt-in vs. opt-out
expressionprivacy
The two default postures of consent: nothing happens until you say yes, versus everything happens until you say no - defaults decide outcomes at population scale.
The single design choice with the largest privacy impact; participation rates swing enormously on it.
Opt-in requires an affirmative act before something applies; opt-out applies by default and requires an act to escape. The distinction sounds procedural and it determines outcomes, because defaults are what most people get.
The empirical finding is consistent and large: participation rates differ enormously between the two, in organ donation, retirement savings, marketing consent and every other domain measured. People are not indifferent, they are busy, and the default is what happens when attention is scarce. That makes the choice of default a design decision with a predictable result rather than a neutral starting position.
In data protection this is why the legal texts specify it. GDPR requires consent to be freely given, specific, informed and unambiguous, which rules out pre-ticked boxes and consent bundled into terms of service. LGPD follows the same shape. The engineering consequence is that opt-in is a system requirement rather than a policy preference: recording what was consented to, when, and for what purpose is a data model decision, and retrofitting it into a system that assumed opt-out is substantially harder than building it in.