Free Will....for the unbeliever

Someone asked me, if you don't believe in a god, how do you then have free will?  I thank this person because it has made me question and think over such a conundrum as this.  Being that free will and determinism are religious in nature, how does an atheist, an agnostic, approach such a topic?

I have a theory.

Without the structure, the foundation of religious beliefs set forth by various authorities over the ages, what do you have?  You have nature.  You have science; but as science is the study of all natural occurrences and objects - invariable you can start a foundation on Nature.  On the biology of species, the physics of atoms and the chemistry of the reactions.  Listings of various and seemingly endless sciences devoted to one subject, on question, after another goes on seemingly forever.

So, where can a concept similiar, if not the same, to free will be placed; be studied?

I stated in the first essay on free will, of a comparison of the conflict between free will and determinism found elsewhere.  This other conflict is where the atheist, the agnostic, the free thinker can go: nature vs nurture.

Are your choices based on genetic sequences mutated over millions of years, demonstrated in your behavior, in the overall behavioral actions of this species?  Or are your choices based on parental, societal, environmental influence affected from the moment of birth, and no earlier?

The theories for the nature cause are great; as are the theories for nurture.

The same thought for the conflict for free will and determinism can be applied to nature and nurture.  I am happy to state that a science writer, Matt Ridley, has already written an excellent book on this very topic with the title: Nature via Nurture.

Just as my reasoning, though based on spiritual beliefs, showed the possibility for free will/determinism via determinism/free will; as is the reasoning for nature/nurture via nurture/nature.

Given the well-thought out and researched subject on the matter of this nature via nurture in Matt Ridley's book; I will not belabor the point; aside from sayign that one should read this book, as well as what other theories have been put forth on these ideas.

However, in ending:

Genetics is influenced by the demands of the environment, fluctuating with the seasons, the hot spells, the ice ages, etc; thus behavior is created.

The behavior influenced by genetics, inherited from the combination of genes passed to child from mother and father; thus behavior is created.

Therefore, for the atheist, the agnostic - the questions are not:
'do you believe you have free will', or
'do you believe your life has been determined'. 
The questions are:
'do you believe that your choices are decided by your genetic makeup', or
do you believe your choices are decided by your environment'.

Or better yet, do you believe your decisions, your chices are influenced by both genetics and the environment in which you were born.