Right--what we saw in, for example, Incredible Hulk #377 when you see the flashbacks to Banner's youth and teen years (with the Savage and Gray Hulks emerging) was really their personalities emerging in realtime except that Banner's body didn't actually change. There were some splits that took place BEFORE he was exposed to gamma radiation...except that may or may not exactly be true if you subscribe to the idea he's always had some gamma radiation in him courtesy of his father's work.
Once upon a time, Peter David tried to suggest that it was Banner's MPD/DID that allowed him to change between forms, as there were at the time very very few other gamma mutates who changed back and forth. Drill further into that and you arrive at the idea that what we know of as "Banner" was in fact not normal, and only thought himself such. That if "Bruce" wanted to, he could use the powers of his other alters while remaining in his most humanoid form. (Quite by accident I'm sure, Bruce Jones arrived at a similar conclusion with his "Super-Banner" incarnation many years later.)What if Banner didn't have these personas when he got the radiation? Would he pretty much just be like the other gamma mutates who pretty much retain their human personas (Ross, Jen and Cho for the most part, and Ewing seemed to kind of imply that their "savage" sides are actually from the "Bruce" radiation they got, as Samson stated). Maybe something like the merged/professor?
The fact Brian Banner was affected by radiation and likely passed some aspect on to his son which may or may not be responsible in full or part for his MPD/DID is one of those chicken-or-egg scenarios for which there is no easy answer. Regardless of that thought, however, writer Paul Jenkins came up with another related theory, namely that Bruce Banner could spawn new personalities and Hulkforms in response to specific traumatic stimuli. Connect this with the above and perhaps that's what he's been doing all his life, until there was enough gamma in his body to actually give them metamorphic substance.
The theory is proven in part if you subscribe to Jenkins' idea that the "Merged" Hulk isn't really a true amalgamation of the extant 3 personalities but an artificially-created gestalt with its own subtle differences from the other personae. In fact, that would mean that Banner is capable of forming new Hulks such as those in the cave in his mind. Clown Hulks, snake Hulks, guilt Hulks, Devil Hulks--Hulks imagined in a moment and locked away in a Darwinian effort of natural selection. Those fittest to survive rise to the top and those who Banner realizes were mistakes being pushed to the bottom until one is given an opportunity by a particularly traumatic event, given light of day as never before.
Food for thought.
~G.