Regarding free will, consider aerial combat (dog-fighting), before the age of air-to-air missiles.
The enemy pilot (Red) comes “out of the sun”, as he is trained to do, and as the friendly pilot (Blue) is trained to anticipate. Blue, upon seeing his adversary, must decide in an instant how to evade Red — if it is not too late to do so. Assume that Blue survives the crucial early moments of his encounter with Red. Blue’s decision about what to do next (probably) will accord with his training; that is, he will choose one of the maneuvers that he was trained in, though he may not execute it in “textbook” style. But the maneuver that he chooses, and how he specifically executes it, will depend on his (very rapid) assessment of the environment (e.g., the enemy’s rate and angle of closure, altitude, presence of clouds, topography), the condition of his aircraft and its armament (e.g., maneuverability, climb rate, ability to withstand the stress of a violent maneuver, accuracy of the machine gun, number of rounds in the magazine, amount of fuel in the tank), and his own confidence in his ability to do what he “should” do, given his necessarily imperfect assessment of the situation, his options, and his ability to exercise each of them.
The key word in all of that is “judgment”. Regardless of Blue’s genetic and behavioral inheritance, he is in a life-or-death situation, and his goal — unless he is suicidal — is to get out of it alive. More than that, he wants to elude Red’s initial onslaught so that he can kill Red. Blue therefore assesses his options with those goals in mind, overriding whatever “instincts” might lead him to panic or choose an inappropriate option, given the specific circumstances of his encounter with Red.
Similarly, but less dramatically, humans make judgments about how they should act so that they can have enough money to buy a house, be healthy, maintain a stable and happy family life, retire comfortably, and so on. The judgments — and the behavior that follows from them — may not “come naturally”: saving instead of squandering, drinking moderately instead of heavily, remaining faithful to one’s spouse, and so on.
Thus I have no doubt that I — and most humans — can (and do) act deliberately in ways that are not strictly determined by genetic inheritance, behavioral conditioning, the moon’s cycle, the position of the stars, or any such influence. (It does seem to me that determinism has a lot in common with astrology.) Determinists bear the burden of proving that freely chosen, purposive behavior is an illusion. (It is no more an illusion than is consciousness.)
Some determinists hew to their faith because it allows them to view criminals as automata who are not responsible for their actions and are therefore undeserving of punishment. Illogically, these criminal-coddling determinists usually favor “rehabilitation” over punishment. That position is illogical because:
If there is free will, punishment can deter wrong-doing and keep wrong-doers out of circulation (for a while, at least). Rehabilitation will work only in those unusual cases where criminals are able to transform themselves, so that their judgments no longer have anti-social consequences.
If free will is lacking (either generally or for persons with certain disorders of the brain), rehabilitation is impossible because criminals are “destined” to commit anti-social acts. But punishment (incarceration or execution) will keep them from committing such acts (temporarily or permanently).
Related reading: “Is Free Will an Illusion?” (a virtual colloquium at The Chronicle of Higher Education)