What I don't see is this other element that enters the equation and somehow causes people to behave in a manner that they understand to be ultimately detrimental to themselves.
What about self-sacrifice? Isn't that pretty self-detrimental?
The person who jumps on a grenade to save his friend. If the grenade went off otherwise, his friend would die, so he'd be the only witness to the action. He'd suffer no social stigma, he'd be alive and able to procreate (potentially), yet he still jumps on the grenade.
Are you saying that he's immediately calculating the potential happiness (or regret) that would come from living versus the vast maw of nothingness that is death, and he's calculating that there is more benefit in the maw?
That just doesn't make any sense to me.