I only disagree with your last sentence. There's no way to perform a careful analysis of option A vs option A+B while maintaining the assumption that they are n

I only disagree with your last sentence. There's no way to perform a careful analysis of option A vs option A+B while maintaining the assumption that they are non-overlapping sets. Simply reading the answers proves that assumption wrong. Thus, it seems way more likely that the explanation is a lack of thinking/analysis... i.e. intuition. People are answering the question they expect instead of the question that's in front of them, because that's easier: http://lesswrong.com/lw/9l3/the_substitution_principle http://lesswrong.com/lw/9l3/the_substitution_principle But you are right, we can't know that from their answers alone. There are numerous possible reasons for picking the wrong answer, so further experimentation is required if we really want to know why.