In my opinion the "nature" vs "nurture" debate is proof that we have a spirit or no free will. The nature side says that we are genetically programmed to act a certain way, that we are who we are due to the organization of atoms in our cells, particularly in our DNA. The Nurture side says that we act the way we do as a response or due to influence of our surroundings. Both sides have plenty of evidence to show that one or the other is more influential.
Consider if only one was responsible for our decisions, say "nature" in that case all of our actions, thoughts etc. would be based on genetics. If this were true I think it would be safe to say that we have no free will but are slaves to our genetics.
Now consider "nurture" where all of our actions were in response to the integration of our surroundings, teachings, examples etc. Again no free will, instead slaves to our perception of the world.
Of course the simple solutions is that we are neither all of one or the other, but instead some of one and some of the other. Still, this does not leave room for free will, because the sum of both concepts leaves no room for free will. Therefore, there must be another factor in our decision making that originates with us but is only influenced by the other factors, or at the very least we can be an originating source of influence on which factor is more powerful at any given moment in our lives.
Interestingly, I presented this concept in a power point presentation to my Genetics class at CSUS with the last slide suggesting we must have a spirit. I don't think it went over very well with the professor, but a lot of the students commented on it positively.