A new development adds a mind-boggling twist to the question of the biological side of the unity of the organism. Toxoplasma infections in the brain can cause rats to lose their fear of cats, which helps the parasite continue its life cycle by infecting cats. It might influence, or in part determine, the personality of humans carrying it. And rates of infection in different nations and cultures, and their change over time, can explain cultural personality aspects such as prevalence of machismo, rates of neurosis or confidence, and interest in novelty.
How much of both our collective cultural personality, and our individual personality, is the sum of influences of organisms dwelling in our bodies? The selfish gene is only the beginning of the reductionism of free will. Sometimes I think the brain's biological purpose isn't to think, but merely to rationalize. Not only aren't we single organisms but committees of billions, even our thoughts, personalities, and identities might, in a very quantifiable sense, be formed in large part by a committee of chemical causes that are determined as much by individual happenstance as by species evolution.
Where, then, is identity? We already knew, if we were willing to, that much of what we consider our free will consists of unpredictable chemical shifts whose causes can be internal and external, and rarely have any relation to the reasons we come up with. All this discovery really does, or could do once it's expanded upon, is expand yet further the gap between what we think our reasons are, and what they may actually be; and add to that gap a new purposefulness, the purpose of an organism that may be shifting our personality merely to allow us to live or die in a place convenient for it to spread. The answer lies not in our stars but in our selves; but is your self your body, or the things that live in your body?