Hard to say; on this board we have many people with spread, but I've been on RSD boards where the numbers are maybe half and half, or there are less with spread; we have more seriously disabled people on this board than the previous incarnation, I'd guess that here the spreaders outweigh the non-spreaders.

I keep seeing similar statements too; really I don't think enough studies have been done. It's important to look at the dates on websites, articles and publications. Far as I can see, for a number of years the same tired old pieces on RSD were being re-hashed and there isn't enough - not nearly enough - emphasis on the fact that lack of research into RSD means that simply not enough is known about it.

But I'm hearing that phrase again - absence of proof isn't proof of absence.

FWIW it hasn't spread yet for me, thank goodness...(yet! yet!), but in general spread seems to me to be on the rise.
all the best