zlacker

I naturally keep my pinky and ring fingers together on my right hand, and I think this is a result of how I hold my phone. It made me wonder if there is an easily overlooked common activity of that time that would cause people to naturally hold their middle and ring fingers together.

Your example of writing makes sense too. Perhaps because writers were a more exclusive club then, prominently showing your subject’s hand with that finger grouping was a message in itself.

>>onioni+(OP)
I’d also consider the possibility that artists might use their own hand as a reference when detailing hands in a painting, and their own physiology might be affected by long hours working with a brush. Artists training other artists might reinforce this collective conclusion that this is a natural relaxed hand pose, because when they look at their own hands that’s often what they see.