Do regular blood tests on a randomized sample of married couples and measure the stress levels.

Once you've found the couple with the lowest overall levels, book them on a touring circuit so audiences can ask them how many silly dances they invented. Since the couple doesn't know whether that's a source of their happiness or not, it won't really get us any closer to an answer. But at least the couple's resulting stress from the tour and impending marital problems will teach the audience about the limits of their method of inquiry.