The most powerful nudge is the default option. When 401(k) retirement plans switched from opt-in to automatic enrollment, participation jumped from 49% to 86% — same people, same plan, same information. Only the default changed.

Organ donation tells a similar story. Countries using opt-out systems reach consent rates above 90%, versus under 20% for opt-in systems. Austria and Germany have similar cultures but very different registration rates — most Austrians are presumed willing, while only around 12% of Germans actively sign up. But consenting is not the same as donating: recent research finds that switching the default does not, by itself, reliably raise actual donation rates, which also depend on hospital capacity, public awareness, and whether families are asked to confirm.

Defaults work because people tend to accept whatever is pre-selected. Changing it takes effort — energy most of us would rather save.