So, about what percentage of your birds have gone broody, even once? Counting only birds that reached adulthood (my birds seem inordinately fond of the Darwin Awards), I'm looking at about 2/3.
Hmm. Probably less than 50% but more than 25% broody in the past two decades, during which time I have mostly had rare or unusual breeds (Dark Cornish, Sumatra, various bantams, etc.)
Before that, I mostly had breeds that were known to be good layers, and broodiness almost never happened (Sexlinks, Rocks, Leghorns, etc.) Maybe 1% broody?
I'm inclined to think that breeds made a big difference, but I don't have a perfect comparison because I don't think I ever had a flock with an even split of both types (breeds selected for high production/non-broodiness, and breeds without that selection).
Thinking back, I wonder how much effect lighting has?
The ones that never went broody were given artificial light in the winter.
The later ones (that included more broodies) had natural light plus whatever might have come from street lights, house lights, and similar sources (not enough other light to affect when they went to sleep and woke up in the morning).