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Studies provide evidence that this difference in participation may occur in part because the requirement to opt-in or opt-out results in large differences in the meaning that individuals attach to participation.
A study by Davidai et al. (2012) examined this phenomena in relation to people's driving licences in Germany and Austria. Respondents who live in Germany, which employs an opt-in donation policy; and conversely, in Austria, they have an opt-out policy.
Can being lazy or not bothered have a bigger impact than you imagine? Well, in Germany: the 'tick box' if you DO want to donate leads to 12% being donors. However, in Austria, a country with a very similar culture and economic development: having a "tick box if you DON'T want to donate" leads to only 1% ticking the box. Consequently, 99% of people in Austria are organ donors!
The placement of organ donation differed significantly depending on whether it purportedly was made in an opt-in country or an opt-out country.