An interesting trend in fountain pen design over the last fifteen years has been the tendency for pens to increase in girth. Pens of earlier eras tended to be much thinner typically than those of the present. So what accounts for this change?

The main theory I will offer to explain this trend is what I call "The life cycle theory of fountain pen widths." It is by now well known that thicker writing instruments are more comfortable to use for people with arthritis and various other joint related problems. Typically these problems arise in older populations. For younger people, thicker pens are still usable, but lead to writing fatigue a bit faster than with thinner pens. Thus, one possible explanation for the trend we see in fountain pen widths is that it reflects an effort by pen manufacturers to customize their products to appeal to an increasingly graying pen population. That is, if the average consumer in a population gets older, the pen appealing to the most consumers will tend to be thicker. On the other hand, when fountain pens were still in common use, the population of users tended on average to be much younger than the current crop of fountain pen users. Hence, a thinner pen offered more appeal.

Here's an alternative theory: Fatter pens are nice to write with for short periods but more fatiguing over long periods. Thinner pens fare worse than thicker ones over short periods but are better for extended writing. If these surmises are correct, then the change in the girths of fountain pens is easily understood as a rational response to the changing use of the pen by an average user. Earlier on, the average user would have more need for a pen that is a good performer over long periods. Now, fountain pens are often reserved solely for signatures and other light work. Faced with this new mission profile, it is optimal to redesign pens to perform well in this role.

The final alternative is a non-theory -- changes in pen girths are purely an artifact of fashion. I think for several reasons this theory is wrong. Here's one such argument -- if pen widths were driven purely by fashion, then we would expect the widths of all writing instruments to vary across time. In fact, there has been very little variation in the widths of pencils and ballpoints. These have remained thin throughout. Notice that their thinness is consistent with a younger generation of users (under the life cycle theory) or no change in their primary mission (extended writing) in the second theory. It seems odd that fashion would only affect the fountain pen segment of the market and not writing instruments more generally.

Given these theories, the prediction is that we can expect pens to remain thick for the foreseeable future.