You are out for dinner and your friend is looking at the wine list and gives you “There’s a house wine and then there’s this Aussie Shiraz that’s supposed to be good, what do you think?”

How you answer depends a lot on how long you have known the person.  If it was my wife asking me that I would not give it a moment’s thought and go for the Shiraz.  If it was someone I know much less about then I would have to think about the budget, I would ask what the house wine was, what the prices were, etc.  Then I would give my considered opinion expecting it to be appropriately weighed alongside his.

This is a typical trend in relationships over time.  As we come to know one another’s preferences we exchange less and less information on routine decisions.  On the one hand this is because there is less to learn, we already know each other very well.  But there is a secondary force which squelches communication even when there is valuable information to exchange.

As we learn one another’s preferences, we learn where those preferences diverge.  The lines of disagreement become clearer, even when the disagreement is very minor.  For example, I learn that I like good wine a little bit more than my wife.  Looking at the menu, she sees the price, she sees the alternatives and I know what constellation of those variables would lead her to consider the Shiraz. Now I know that I have a stronger preference for the Shiraz, so if she is even considering it that is enough information for me to know that I want it.

Sadly, my wife can think ahead and see all this.  She knows that merely suggesting it will make me pro-Shiraz.  She knows, therefore, that my response contains no new information and so she doesn’t even bother asking.  Instead, she makes the choice unilaterally and its house wine here we come.  (Of course waiters are also shrewd game theorists.  They know how to spot the wine drinker at the table and hand him the wine list.)

In every relationship there will be certain routine decisions where the two parties have come to see a predictable difference of opinion.  For those, in the long run there will be one party to whom decision-making is delegated and those decisions will almost always be taken unilaterally.  Typically it will be the party who cares the most about a specific dimension who will be the assigned the delegate, as this is the efficient arrangement subject to these constraints.

Some relationships have a constitution that prevents delegation and formally requires a vote.  Take for example, the Supreme Court.  As in recent years when the composition of the court has been relatively stable, justices learn each others’ views in areas that arise frequently.

Justice Scalia can predict the opinion of Justice Ginsburg and Scalia is almost always to the right of Ginsburg.  If, during delibaration, Justice Ginsburg reveals any leaning to the right, this is very strong information to Scalia that the rightist decision is the correct one.  Knowing this, Ginsburg will be pushed farther to the left:  she will express rightist views only in the most extreme cases when it is obvious that those are correct.  And the equal and opposite reaction pushes Scalia to the right.

Eventually, the Court becomes so polarized that nearly every justice’s opinions can be predicted in advance.  And in fact they will line up on a line.  If Breyer is voting right then so will Kennedy, Alito, Roberts, Scalia, and Thomas.  If Kennedy is voting left then so are Breyer, Souter, Ginsberg, and Stevens.  Ultimately only the centrist judges (previously O’Connor, now Kennedy) are left with any flexibility and all cases are decided 5-4.

When a new guy rotates in, this can upset the equilibrium.  There is something to learn about the new guy.  There is reason to express opinion again, and this means that something new can be learned about the old guys too.  We should see that the ordering of the old justices can be altered after the introduction of a new justice.  (Don’t expect this from Sotomayor because she has such a long paper trail.  Her place in line has already been figured out by all.)