Writing is another example [of flexible habits]. The way I hold my pen and form my letters is entirely habitual. I could hardly alter my style of handwriting, however much I tried. It has become a part of me that I and others ... have to live with. ... There are also other habits involved, My style or my lack of it, which could be as much something that is tenth nature to me as my handwriting. But it is not quite automatic writing, like automatic sleep walking; the content of what I write is not just habitual. Habit does not enter in ... I can harness the collection of [my] habits to some new use, and attempt to vary it (p. 90).Habits "automate" behavior and allow us to concentrate on things other than the automated activity. This is clearly true of writing.
It appears to me that this explains—at least to some extent—why many writers report that there is a significant difference between handwriting and writing on a keyboard. Thinking might seem to flow more easily and you might seem to be more creative when you rely on a firmly ingrained habit. Hell, you might be more creative when you don't have to think about how you put thought on paper (or some other medium). However, if this is true, it also means that there is no intrinisc cognitive difference between handwriting and keyboarding, between using a fountain pen and a pencil. It all depends upon what you are used to. Apparent differences can be traced to habit.
I am sure that this "hypothesis" could be empirically tested. I am just as sure that I cannot test it, however.