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I've never really looked into exactly what a parallel universe is. I once heard someone define it as this:
Every time a decision is made or something happens, the world splits off into a number of parallel universes: one for each possible decision/occurence.
Is that what you mean?

Doesn't the existance of parallel universes depend on free will?

Well... I think the answer is no. The reason an asteroid strikes one planet instead of another is due to a series of events leading up to the actual occurrence.

Every effect can be traced back to a cause. That cause can be traced back to a prior cause. This can be traced back to the Big Bang. (The real causality question here is what caused the Big Bang?)

I tend to think that this is the same way with any human decision. Every time we act, every single thought we have, is influenced by a previous act or thought.

This is why I have a hard time accepting that we have free will...

And if you don't accept free will, then you can't accept that a fly could possibly land on me instead of you, thus creating a parallel universe.

Ooo, a free will conversation.

In my view, if we accept a traditional understanding of time, leading to a linear cause and effect then we are always in a perpetual state of reaction to other causes. ( Our reaction in turn produces more effects, which in turn produce more causes. ) As you say, we do learn from experiences. Every experience that we have can in turn be shown to have a cause. Missed the city bus at 7:00 and so had to walk to work. Then you never would of been mugged. You missed the bus because you stayed out late last night. You stayed out late last night because a friend was in town. Etc. etc. Ultimately, assuming every effect has a cause, we can trace back our current action all the way down the time line of our life. Our first reactions were determined by our genetic makeup and so that could be considered our first cause. ( Though everything that caused our parents to make us could also be in a sense a cause. ) So in a sense, I am basically agreeing with Katty.

There are some assumptions made by the above though. ( There are probably other holes in it I'm sure. ) Linear time, cause and effect is a necessity. Also, if the quantum universe is probabilistic, such small perturbations can lead to dramatic results over the years, through chaos theory and the butterfly effect. ( Also a probabilistic quantum universe could lead to the parallel universes on its own. Little, cute, decision making electrons. )

Does such a cause and effect model degrade in any way living action? We still have to react, and have the ability to generate new causes. It just so happens that our causes are caused.