Why do people become more set in their ways (or more conservative, in the non-political sense of the term) as they get older? I think the popular explanation is the “rusty brain” theory: people’s minds just start to get crusty and hard to move as time passes, like rusty machines in need of oil. They resist change because friction in the gears impedes free thinking and hence the acceptance of new ideas. Perceived this way, resistance to change is a form of irrationality that derives from inefficient mental processes.
Naturally, I have an alternative theory. Resistance to change is the outcome of rational information processing. Think of people as Bayesian updaters: they have prior beliefs about the world -- essentially probability estimates -- which they update on the basis of a stream of new information they receive from their environment. They engage in hypothesis testing, and whether any given hypothesis is accepted or rejected depends on the data set. A young person has a small data set, because he has had few experiences and less time to observe the world. Consequently, the marginal impact of any new observation is larger. An older person, on the other hand, has a large data set, and therefore the marginal impact of one additional data point is likely very small. (If I’ve only had nine experiences of a particular kind so far, one new event constitutes 10% of my experiences of that kind, and the effect on my beliefs could be quite dramatic. If I’ve had 99 experiences of a particular kind, one new event constitutes a mere 1% of my experiences of that kind, so radical shifts are unlikely.) It follows that rational people will tend to become more obstinate in their beliefs over time.
Of course, I’m leaving out many potential sources of irrationality here. For example, it is entirely possible -- likely, even -- that one’s current beliefs shape both the collection and interpretation of new observations. The result is a path-dependent process in which new observations systematically tend to reinforce the conclusions that resulted from old observations. I’m not saying this doesn’t occur; I’m merely pointing out that irrationality is not necessary to explain the resistance of the older people to change.
I have another, complementary theory of the same phenomenon, but I’ll wait until later to post it. Jim also has a theory that he described at lunch today -- something about human capital -- so maybe this post will prod him to post his ideas as well. (When I invited Mike and Jim to join the blog, we agreed not to put each other on the spot like this, but this time Jim gave me permission to do so.)