Back in the character-based Stone Age of the personal computer, all IBM-compatible MS-DOS PCs started up with a C-prompt, the “C:\>” text with a blinking cursor. At least, all PCs that weren’t broken.
Can we conclude anything from that? That “C:\>” is a reflection of some supernatural or transcendental truth? That it is an insight into God’s mind? No—it’s just a useful trait shared by this class of PCs. There’s no objective meaning behind these characters. This text is useful (it shows the directory in which any typed commands will take place), so it was selected. There’s nothing more profound than this behind it.
Human morality is like this. Almost all humans have shared moral instincts, not dissimilar from instincts in other animals. Through instinct, honeybees communicate where the nectar is, newborn sea turtles go toward the ocean, and juvenile birds fly. Training or acculturation can override human instincts, of course, but in general we have a shared moral sense—a shared acceptance of the Golden Rule, for example.
We think our moral instincts are pretty important, and that’s understandable, but there’s no reason to imagine that they are objectively true—that is, based on some supernatural grounding. Said another way, we think that our morality is true because it tells us that it’s true, but we can’t infer from this that it is grounded outside us.
We must not confuse universally shared moral instincts with universal moral truths.
Human moral instincts are what our programming says they are—it’s no more profound than that. There’s as much reason to imagine that they are a window into the transcendent as that the MS-DOS C-prompt is.
Related posts:
- More on how our instincts are well tuned to the environment: Plantinga’s Nutty Evolutionary Argument Against Naturalism
Galileo Unchained 
Morality, as a matter of pure opinion, is entirely subjective. It’s only the fact that it has an evolutionary basis that almost all humans share that makes it seem anywhere close to universal.
But, as has often been observed, if men could get pregnant, abortion would be a sacrament. And, if we were all descended from mantises instead of apes, it would be a mortal sin NOT to eat your husband after sex.
Sheesh. Why not leave morality definitions to the philosophers? You don’t know what you are talking about.
Tell me more. What is missing or unexplained from the hypothesis that human morality is a combination of moral instinct and moral customs from society?