Behavioral geneticists (BGs) don’t like to be called “genetic determinists.”  “No, no, no,” they protest, “all we’ve shown is that genes exert some influence.  Twin and adoption studies show that environment is important, too.” 

But what would they say if you replied, “OK, so you’re a genetic + environmental determinist.  Same difference”?

Most BGs would probably reluctantly accept the charge.  After all, what else is there besides genes and environment?

If you take a closer look at BG research, though, you’ll notice something interesting.  Virtually every BG study partitions variance into three sources: genes, shared family environment, and non-shared environment.  Typical estimates are something like 40-50% for genes, 0-10% for shared family environment, and 50% for non-shared environment.

And what exactly is non-shared environment?  Everything other than genes and family environment!

Why do I bring this all up?  Well, suppose human beings had real, honest-to-goodness free will.  If it made a difference for behavior, where would it show itself?  In the BG framework, it would be filed under “non-shared environment.”

OK, now let’s get Bayesian.  If you could fully account for a person’s choices using genetics and measurable environmental variables, you’d count it as a confirmation of determinism, right?  Well, if you buy this argument, you also have to buy its mirror image: The harder it is to account for a person’s choices using genetics and measurable environmental variables, the stronger the case for free will.

From this perspective, the large empirical estimates of the importance of non-shared environment are noteworthy.  Identical twins raised together are still, in many ways, very different.  The believer in free will can simply say, “The good twin and the evil twin just made different choices.”  The determinist, in contrast, can only ask for a blank check: “One day, we’re figure out the hidden forces that caused them to be so different.  Until then, bear with us.”

Let me hasten to add that the magnitude of non-shared environment in BG research is not the main reason why I believe in free will.  In my view, free will is a fundamentally a modal claim, not a predictive one.  You can know with overwhelming certainty that I won’t shave my head tomorrow; it doesn’t change the fact that I could shave my head tomorrow.  And the main reason why I believe in free will is introspection, not any fancy argument. 

Nevertheless, I strongly suspect that if non-shared environment’s contribution to behavioral variance were a lot smaller, determinists would be heralding the result as “proof” of their position.  And if this suspicion is right, it’s only fair to ask them to reduce their confidence in light of the findings of BG research.