In fact, the truth is surprisingly simple: much depends merely on what happens if people don't make a decision, something called a no-action default, or simply a default. The countries on the left of the graph ask you to choose to be an organ donor, and those on the right ask you to choose not to be a donor. If you do not make an active choice, you are, by default, a nondonor in Germany and a donor in Austria.

Dan and I wanted to understand this. We started by asking a sample of Americans whether they would be donors or not by presenting them with a choice on a webpage. One group, the opt-in condition, was told that they had just moved to a new state where the default was not to be an organ donor, and they were given a chance to change that status with a simple click of a mouse. A second group, the opt-out condition, saw an identical scenario, except the default was to be a donor. They could indicate that they did not want to be a donor with a mouse click. The third group was simply required to choose; they needed to check one box or the other to go on to the next page. This neutral ques-tion, with nothing prechecked, is a mandated-choice condi-tion; it's important, because it shows what people do when they are forced to choose.

The effect of the default was remarkably strong: when they had to opt in, only 42 percent agreed to donate, but when they had to opt out, 82 percent agreed to donate. The most interesting result was from those forced to make a choice: 79 percent said they would be a donor, almost the same percentage of donors as in the opt-out condition. The only difference between the group that was asked to opt out and those who were forced to make a choice was that we forced the respondents in the mandated-choice condition to pick either box before they could go forward. It shows that if forced to make a choice, most participants would become donors. Otherwise, if they were given a default, most simply took it, whatever it was.

From The Elements of Choice by Eric Johnson

It's more complicated than the one example, and he covers it further, but as a rough guideline, it looks like forced choice and opt out are similar in this case. Which would make sense because the opposition is mostly religious and strict religious people are more motivated to opt out.