Any parent knows that toddlers are constantly bashing thier heads off things because they’re top heavy and learning to walk they constantly fall over and tumble off things. One of my daughters used to constantly have a lump on her head from smacking it off something. My friends son used to run full tilt into things like brick walls given half a chance, the noise it made was awful. I stand to be corrected on this but if I remember rightly the male scientist who came up with shaken baby syndrome didn’t have kids and hadn’t considered the possibility that these types of injuries could be self inflicted.
I don't know if Norman Guthkelch and John Caffey had kids, but Caffey wrote in his 1972 paper:
> Rhythmic whiplash habits of the infant himself during the first months of life, such as head-rolling, body-rocking, and head-banging may be traumatically pathogenic to his brain and its veins.
More generally, Caffey considered many types of mild shaking to be potentially dangerous, including "toys and recreational contraptions", "playful practices" etc. The notion that shaking had to be "abusive" to be pathogenic came in the 10 years afterwards.
