The question of free will is indeed an intriguing one. From one perspective, and from our own personal experience it would appear as though we have free will, but from a more objective perspective it certainly appears from our collective actions as if we don’t have free will.
I would liken it to shining a flashlight on a wall. We know from experience that the light will form a specific pattern on the wall, with more light falling toward the center of the beam and less light the further that one gets from the center. Science would refer to this as probabilistic behavior, there’s a higher probability that the individual photons will arrive at the center of the beam than there is that they’ll arrive at the periphery. And while it’s impossible to predict where each individual photon will end up, it’s highly predictable as to where the photons will collectively end up. Behavior which is completely unpredictable on an individual basis becomes highly predictable on a collective basis.
Now the thing is that our human behavior follows this exact same pattern, with behavior seemingly unpredictable on an individual basis, but far more predictable on a collective basis. Take for example the emotionally neglected child to which the OP referred, we may not be able to predict which of the children will grow up to be sexually promiscuous, but we know that a greater number of them will. Collectively, their behavior is simply probabilistic. And the more that we know about the initial conditions, the more accurate those predictions will be. One might argue that each individual freely chooses which path to take, but we can predict at the outset the number of them that will choose each path. The free will of the individual will inevitably lose out to the probabilistics of the collective. Like the photons from our flashlight, we don’t know which individual will end up where, but we know how many will end up at each point, and free will doesn’t seem to be able to change that.
So if we look collectively at the behavior of human beings, it looks probabilistic. Some of them will be more like sinners, and some of them will be more like saints. Some of them will believe in this, and some of them will believe in that. Some of them will be passive, and some of them will be aggressive. The initial conditions determine the ultimate outcome. It’s simply probabilistics. Probability, not free will determines how many will end up in heaven, and how many will end up in hell.
To prove that free will actually exists one must get the collective to do something which the initial conditions predict that they shouldn’t do. It’s not our individual choices that prove or disprove free will, but rather our collective choices. When the photons can act in a manner that defies probability, then we’ll know that they have free will, and the same is true for us. When people collectively do what probability predicts that they shouldn’t do, then we’ll know that they have free will.