Making organ donation the default (opt-out) hugely raises consent.

Johnson & Goldstein (2003) compared European countries with opt-in donor registration (you must actively sign up) to those with opt-out (you are a donor unless you decline). Effective consent rates were dramatically higher under opt-out — often above 90% versus well under 30% in similar opt-in countries. Because most people stick with the default, the design of the form quietly determines the outcome. This is a landmark demonstration of the power of defaults, central to 'nudge' policy.

The nuance is important: registered consent is not the same as transplants performed. Actual donation rates also depend on hospital infrastructure, family consent practices, and how the opt-out is implemented, and some studies find the effect on real transplants smaller than the consent figures suggest. Still, the core claim — that defaults strongly shape donation decisions — is well supported.