Certainly, human nature is fixed. It's universal and unchanging — common to every baby that's born, down through the history of our species. But human behavior — which is generated by that nature — is endlessly variable and diverse. After all, fixed rules can give rise to an inexhaustible range of outcomes. Natural selection equipped us with the fixed rules — the rules that constitute our human nature. And it designed those rules to generate behavior that's sensitive to the environment. So, the answer to 'genetic determinism' is simple. If you want to change behavior, just change the environment. And, of course, to know which changes would be appropriate and effective, you have to know those Darwinian rules. You need only to understand human nature, not to change it.