Whether you're an organ donor often hinges on a single pre-ticked box

Countries where citizens are automatically donors unless they opt out have far higher donor consent rates than opt-in countries, even though people's actual wishes barely differ. Psychologists Eric Johnson and Daniel Goldstein showed the same effect in controlled experiments: whichever option is the default is the one most people keep, because opting out takes effort most never make.

— Eric J. Johnson and Daniel G. Goldstein, Do Defaults Save Lives? — Science, vol. 302, 2003