There was a study around 2013 by anthropologist Jamie Tehrani, who concluded that "Little Red Riding Hood" and "The Wolf and the Kids" are two descendants of a common ancient ancestor.
This is an example of monogenesis. I've been reading a little about polygenesis and monogenesis, after someone mentioned them on a site I frequent. As it relates to folktales, polygenesis means the tale originated from many sources independently and spontaneously, and monogenesis means it originated from one source and was diffused.
I think I can safely say that the German Daumesdick, the Russian Malchik-s-Palchik and the Italian Cecino are all the exact same story. This story appears frequently, under different names, across Europe, Asia, and North Africa. Further south in Africa and on other continents, it doesn't seem to show up at all until colonists brought it there. It's pretty clearly one story that was diffused around a specific landmass.
Where it gets more interesting is examples like the English Tom Thumb and the Japanese Issun-Boshi. These tales are from opposite points of the Thumbling tale area, and were first published far before the others. They share a few points: the wish for a son miraculously granted, the tiny boy leaving home, wielding a needle as a sword, serving a nobleman, and being swallowed by a beast. In every other aspect, they differ completely.
Issun-Boshi was probably first written down sometime during the Muromachi period (1392-1573), and there were other stories of tiny people, like the god Sukuna-biko, first described in the 8th century, or Princess Kaguya, first written down in the 10th century.
There's evidence of a Tom Thumb tradition in England as early as 1579.
Portuguese traders reached Japan in 1543, so there's contact between Europe and Asia at that point.
Did these stories have the same source, or were they simply examples of a universal interest in tiny people? Did some country somewhere in the middle produce the proto-Thumbling tale?
I was interested in the same stories appearing across different cultures long before I got onto this project. For instance, the story of a great flood as punishment from a deity (usually with only a handful of survivors in a boat) is universal. It's in the Middle East. It's in Europe. It's in Asia. It's in Africa. It's in the Americas. It's in Australia.
Is this polygenesis or monogenesis? Is it just because all of these cultures had lived near water and seen catastrophic floods, and they wanted to tell stories about them, and the folktales emerged in a kind of convergent evolution? Or could these stories possibly serve as evidence for a singular Great Flood?
Some proponents of polygenesis in folktales based it on a theory of psychic unity among humans, and the idea that all cultures went through exactly the same stages of development. The term "universal human psyche" tends to pop up in some of these sources.
Proponents of monogenesis have also come up with some weird theories. There was a period of time where quite a few scholars posited that all stories were connected. If there was a similar name, it was the same person. In many cases, this was leaping to conclusions. For instance, not all characters named Thomas are the same person. Tom Thumb is not Tam Lin. I think, as time has passed, this particular theory has fallen out of favor.
Overall, monogenesis and diffusion seem far more likely to me, although there are still cases for different groups coming up with the same idea. Like, say, a sky god who controls the thunder.
Researching folktales and fairies, with a focus on common tale types.