Below is an excerpt from the Social Sciences Statistics Blog regarding the differences in countries who have a opt-in versus and opt-out policy on organ donation. Here are links to the original paper.
The results on organ donation in Europe are particularly striking, as the authors show that large differences in organ donation rates in otherwise similar European nations (e.g. Sweeden and Denmark) may in large part be a consequence of whether organ donation is an opt-in or opt-out option on the drivers license application.
As the authors note, there are substantial public policy implications to research along these lines. For example here in the U.S., there is a growing chorus of policy gurus, including at least one major presidential candidate, pushing for policies such automatic retirement accounts. The idea is that rather than enacting more blunt mechanisms (e.g. mandates), we can implement policies that harness the inertia brought about by default options to achieve policy goals.