Abstract
Many of today's pressing societal challenges, such as organ shortages, low vaccination rates, and climate change, require significant changes in individual behavior. One promising intervention to encourage such behavioral change is the opt-out default, which presumes consent for a desirable action rather than requiring active opt-in. While past research focused on the impact of opt-out defaults on the targeted behavior, potential crowding out of related behaviors has been largely overlooked. Here, we investigate whether adopting opt-out policies for deceased organ donation reduces living donations, a related prosocial behavior serving the same public good. Analyzing epidemiological panel data from countries that adopted an opt-out default between 2000 and 2023, we find that the policy switch, on average, leads to a nonsignificant increase in annual deceased donor rates of +1.21 people per million population (+7%, P = 0.213) but to a significant decrease in living donor rates of -4.59 people per million population (-29%, P = 0.026). Across four additional studies, we demonstrate that this crowding-out effect is reflected in a reduced willingness for living altruistic (vs. familial) donations and is attributable to a stronger belief that the organ supply is sufficiently met with deceased donations under opt-out (vs. opt-in). Our research advances insights into the unintended consequences of default nudges and suggests ways to mitigate them.