Consider trying to predict the details of unattached people’s kisses. That is, you might have data on who such people have actually kissed when, where, and how, and data on who they say they would be willing to kiss under what circumstances. From such data you make models that predict both the kisses that actually happen and the kisses they say they are willing to join. For example, you may notice that they kiss more when they are awake, are not busy with other activities, and feeling frisky. They kiss more when they and their partner are clean and well groomed. They kiss more when they are more attractive to others, and when other willing partners are more attractive to them according to their preferences.

Now consider doing the same exercise for people who are married. When you fit this sort of data, you will find one new big factor: they almost always kiss only their spouse. And if you try to explain both these datasets in the same terms, you’d have to say spouses are in some strange way vastly more attracted to each other than they are to everyone else. This attraction is strange because it isn’t explained by other measurable features you can see, and no one else seems to feel this extra attraction.

Of course the obvious explanation here is that married people typically make a commitment to kiss only each other. Yes there is a sense in which they are attracted more to each other than to other people, but this isn’t remotely sufficient to explain their extreme tendencies to kiss only each other. It is their commitment that explains this behavior gap, i.e., this extra strong preference for each other.

Now consider trying to predict policies and public attitudes regarding limits on who can migrate where, and who can buy products and services from where. And consider trying to predict this using the foreseeable concrete consequences of such policy limits. In principle, many factors seem relevant. Different kinds of people and products might produce different externalities in different situations. Their quality might be uncertain and depend on various features. One might naturally want a process to consider potential candidates and review their suitability.

Such models might predict more limits on people and products that come from further away in spatial and cultural distance, more limits on things that have lower quality and higher risks, and more limits when there is more infrastructure to help enforce such limits. And in fact those sort of models seem to do okay at predicting the following two kinds of variation: variation on limits on people and products that move between nations, and variation on limits on people and products that move within nations.

However, if we compare limits between nations and limits within nations, these sort of models seem to me to have a big explanatory gap, analogous to the kissing attractiveness gap in models that predict the kisses of married spouses. Between nations, the default is to have substantial limits on the movement of people and products, while within nations the strong default is to allow unlimited movement of people and products.

Yes, the context of movement between nations seems to be on average different from movement within nations, and different in the directions predicted to result in bigger limits on movement. At least according to the models we would use to that explain such variation between nations, and variation within nations. But while the directions make sense, the magnitudes are strangely enormous. A similar degree of difference within a nation results in far smaller limits on the movement of people and products than does a comparable degree of difference between nations.

We are thus left with another explanatory gap: we need something else to explain why people are so reluctant to allow movement between nations, relative to movement within nations. And my best guess is that the answer here is another kind of commitment: people feel that they have committed to allowing movement within nations, even if that causes problems, and have committed to being suspicious of movement between nations, even if that makes them lose out on opportunities. That is part of what it means to have committed themselves to by joining a nation.

If this explanation is correct, it of course raises the question of whether this is a sensible commitment to make. For that, we need a better analysis of the benefits and costs of committing to joining nations, an under-explored but important topic.