I took the following off the web from http://www.familycompanions.com/index.html. Read that, then comes my theory about them.
“Ragdolls originated here in Southern California in the early 1960’s and were created by a breeder named Ann Baker. There are many myths that surround the development of the Ragdoll breed, but the myth that seems to be the most popular is: A pregnant cat named Josephine, (Persian-Angora cross) was hit by a car somehow altering the DNA of her unborn kittens. These kittens seemed to exhibit unusual and different characteristics from other cats. They tended to be much more relaxed. In fact, they were so relaxed, when picked up they just hung there much like a rag doll. Hence the name Ragdoll came to be.”
So here’s my theory. When the mother cat was hit, opiate-like pain killers flooded into her blood and consequently into unborn kittens, permanently affecting their psychology and behavior. The following scientific explanation may help, also taken from the net:
“Once the brain perceives the pain, the body releases neuromodulators, such as endogenous opioids (endorphins and enkephalins), serotonin, norepinephrine, and gamma aminobutyric acid. These chemicals hinder the transmission of pain and help produce an analgesic, pain-relieving effect. This inhibition of the pain impulse is called modulation.”
The kittens were modulated – acutely and permanently. They were born as if on opiates and retained the behavior those modulating chemicals produce.
Here is a blog with a lot more on Floppycats.