Why does a pre-ticked box change what we choose?

Because doing nothing is the easiest choice, and the default is what you get when you do nothing. Making an option the default reliably makes people more likely to end up with it. The most striking example is organ donation. In opt-in countries, you are not a donor unless you register. In opt-out countries, you are a donor unless you ask to be removed. Germany uses opt-in and has a consent rate of 12%. Austria, with a very similar culture and economy, uses opt-out and has 99.98%.

Why does a pre-ticked box have so much pull? Several forces stack up. Weighing options takes effort, so people follow a simple rule: if there is a default, leave it. Switching can cost time or money. The default becomes a reference point, so leaving it feels like losing something. And people read it as a quiet recommendation from whoever set it. Oddly, telling people why a default was chosen doesn't weaken its effect.

This is the most famous tool of "nudging", popularized by Richard Thaler and Cass Sunstein's 2008 book Nudge. But the story has limits. Consent on paper isn't the same as organs actually donated: opt-out countries range from 40.2 donors per million people in Spain to just 6 in Greece, and Spain's own transplant chief says the law probably isn't the main reason for its success. And nudging as a whole is contested: after correcting for publication bias, one analysis found no evidence that nudges work on average, though skeptics agree some, like defaults, can be highly effective.