I don't know the origins of the first knuckle method but I suspect it's ancient, probably from China where it is believed rice was first cultivated. People have been cooking rice for several thousands of years, long before cup measurements and electric rice cookers were available, so no doubt something as practical as using one's first knuckle was the preferred method.
I know that people are different sizes and one person's first knuckle is a different measurement than another's, so it's a little puzzling how this can be a universal method. Yet I know it works for me and my considerable shorter/smaller friend and her 6-foot tall husband. Bottom line, it works!
I've also seen a variation on this where the rice is poured to your first knuckle and the water to your second. In some ways, this makes more sense to me because it's about the ratio of water to rice which is more important that measurements.
Do you use the first knuckle method?
Related: What is the Best Way to Cook Rice?
(Image: Dana Velden)