But if we're talking about why people make certain decisions as opposed to very similar but alternate decisions, it's deterministic to say that you only ever make decisions because your brain chemistry demanded that you do so.
But that's not what I'm saying. In fact, it's the exact opposite of what I'm saying. My point is that, if two timelines diverge because of a quantum-level change in a subatomic particle in your brain, that probably
won't affect your decisions, because your decision-making processes take place on a higher level of neural organization and are influenced by a wide variety of macroscopic, classical factors rather than being a deterministic result of quantum states in the brain.
For instance, a lot of the fictional discourse about alternate histories says that if you turn right at an intersection in one timeline, you may turn left in another. But that ignores the fact that you'd usually have a specific reason to turn right at the intersection because you're going somewhere in that direction, rather than it just being a flip of a coin. A change in the quantum states of your brain as you waited at the intersection wouldn't alter your reasons for turning right, so you're probably going to turn right in every single quantum timeline. Even if you are just driving around aimlessly, your "random" choice to turn right instead of left could be influenced by numerous external factors you're not aware of, like traffic patterns or a subliminal cue in the lyrics on your CD player or the fact that the sky looks more inviting to the right, or by the flow of hormones in your brain and body which are as much a macroscopic process as the flow of traffic. It would require a very strange and improbable set of circumstances for a quantum-level change to alter such macroscopic processes to the extent that you end up turning left instead, so even if alternate timelines diverge all the time, you'd still turn right in the vast majority of them (though we can't absolutely rule out the occasional rare exception, of course).
So, bumping at the limits (for now) of scientific testing, we don't know whether you might make a different decision in a different quantum reality (even if you decision is not the quantum split that generates that reality), so it's facile to definitively state that you would not, because we don't know enough about the processes that drive decision making (or, indeed, if Data had a soul or not).
What??????? Where in the hell are you getting "definitively" from? Here is what I actually said (emphasis added and spelling corrected):
But people's decisions and actions, and events like coin flips or dice rolls, are macroscopic events that follow classical physics and wouldn't likely be affected by changes on the quantum level, except in unusual cases. So it's possible that most parallel timelines would be essentially indistinguishable from our own except on the level of a few subatomic particles here and there.
I don't know how you could read those words and imagine that I was asserting anything as "definitive." I'm speaking in terms of probabilities and reasonable conclusions, but I'm not ruling anything out absolutely, because any honest, rational observer must acknowledge areas of uncertainty. All I'm trying to do is to warn against the tendency to assume that the fictional take on alternate timelines -- the idea that they result from different choices -- is "definitively" correct.