The only two plausible explanations to me are either that <i>artists</i> conventionally drew hands like this (for religious, artistic or other reasons) or that artists' <i>subjects</i> conventionally posed like this, for a similar variety of reasons, or because the artist told them to.<p>The article helpfully rules out a third explanation, an "epidemic of syndactyly", but doesn't make a strong decision between the other two. It seems to lean towards this being a quirk of the artists, but it could do with a quantitative study: if artist A painted subject A like this, what happened when artist A portrayed other subjects, or other artists portrayed A?