I think I've been skirting around this one for a long time, never really putting it out there what I suspect is true of sin and sinfulness. So let's go for it.
I think that humanity has been created with a certain epistemology, a way of knowing things. We learn about the world through stimulus and response and eventually our understanding grows more complex until we develop a consciousness. We come to the conclusion that there is an I in the midst of the neural network, a thinking thing, a centre of thought, a ghost in the machine. Consciousness arises from the brain and the brain develops through sensory experience.
We can't help but put ourselves at the centre of the universe because for all we know, we are the centre of the universe. We don't know for sure that anyone else has a mind, even though we act as though they do. Everything that happens, happens in relation to ourselves as the cosmic centre. Even space-time is defined this way. Up: defined in relation to ourselves. Before: defined in relation to ourselves. Later: defined in relation to ourselves.
In order to function as creates in the universe, we need to first experience the universe and to formulate a consciousness at the centre of that experience. Our primary experience is of self-centredness. Our natural condition is self-centredness. Our inclinations are self-centredness. We do what is easy or pleasurable. We avoid what is difficult or painful. We act out of self-interest, an interest which is developed through our previous experience of the world. In order for us to be conscious, we need to develop a self-centredness. Once we have this, then we are free to make choices.
Unlike a Chalmers-esque zombie, there is an I that arises from my brain and allows me to truthfully make the statement: I choose. There is an I and it is an I that chooses. In order to have free will we must be aware of the I that exercises such free will. If we are to be able to make a choice to follow Jesus or not, we must first be able to choose. To be able to choose we need to have a sense of self, a self-centredness. Self-centredness is the natural outcome of a biology that channels all sensory experience to a single brain.
This biology is the result of evolutionary processes, set in motion by a creator God. God has created these processes and the pure freedom of the individual is the result. This means that there was no Fall, there was only an exposure of our true nature. It wasn't as though we started as perfect and later became sinful. Instead we started as sinful and every time we have a choice to make, it exposes our natures.
So what is sin? Sin is the epiphenomenon of consciousness: selfish indulgence.
To say it again: stimulus-response creates a centre of the self; the self-centre creates a consciousness; consciousness allows free will; free will usually chooses self-indulgence through avoidance of pain or pursuit of pleasure.
This, I think, is an explanation of sin that doesn't rely on myth or parable, but which derives from what we know about the human brain today.