Isn't Morality Just a Social Construction?

Just had a great meeting with one of my buddies from my part-time job in retail.  We have been working through Keller's The Reason for God: Belief in an Age of Skepticism and talking about the chapters.  He says that he is an agnostic.

Today we talked about morality and one of his challenges for me was his claim that there is no universal morality. He believes this points to the fact that there can't be a God who is the ultimate standard for right and wrong.  He cited the fact that many different cultures throughout history have defined morality quite differently.  Doesn't that point to morality simply being a construction of different societies?

Greg Koukl has some great responses to this line of thinking.  He writes:

Even if relativists are right that cultures differ radically in their basic moral values, so what? The observation in itself proves nothing, because no conclusion about morality follows. Just because cultures differ on moral viewpoints doesn't mean that objective moral truth is a fiction. In logic this is called a non-sequitur; the conclusion doesn't follow from the premises.

Observations about the practices of groups of people, even if accurate, don't translate into valid conclusions about the true nature of morality per se. How does it follow that because each group thinks it's right, therefore no group is correct? The simple fact of disagreement on morality doesn't lead to the conclusion there is no moral truth. This confuses the epistemological question (the accurate knowledge of objective values) with the ontological question (the existence of objective values).

Currently there are conflicting views on many things. The fact that there is disagreement, however, doesn't mean that no view could be correct. The same is true with differences of opinion on morality.

And in another article he writes:

Why can't somebody experience ethical guilt for something they're doing wrong in different cultures? Maybe morals are really relative. This is descriptive relativism. It entails that you can look at different cultures and find different ethical practices. How does one account for that if there is some absolute morality? It seems that morality is culturally determined.
Here is an observation that may help. It's a refinement, a distinction that will help make sense of some of that. For example, in India they used to practice "settee", which means that when a man died they would also kill the widow. They would bury the wife with the husband. They considered that moral, but we don't consider it moral in our country, so it seems to be a different value on human life.

The refinement that I'd like to offer you is the idea of the difference between facts and values. In India there was still the same value as in the United States that one should not murder. It is not open season on human beings in India. They believe that you should not murder and in our culture we believe you should not murder. That was the same value, but they had different notions of what murder entailed. For example, if you are attacked by somebody with a knife and you kill them, that would be a traumatic experience for you, but you would not have a sense of ethical guilt because you did not murder that person. You killed in self-defense. Therefore, in our culture we are taught that killing in self-defense is not inappropriate or ethically wrong. Having said that, we are not making it open season on everyone because there are certain conditions in which killing is not murder. Those are the facts of killing in our culture. In the same way, in India killing by settee was not considered murder. It was considered a gracious thing to do given certain circumstances.

The reason that I make this distinction is because a lot of times the apparent differences of moralities in different cultures is really not a difference in values but a difference in facts, a difference in how they view certain circumstances. I don't think any culture thinks that child abuse is good, but some cultures are rougher on their children than others. They don't consider that particular roughness an example of abuse. That may help in terms of the distinction of the cultures.

Even so there still is that moral machine and what I'm replying to is not making an argument based on a standard morality that everyone has. I'm responding not so much to the content of morality as to the machinery of morality. We have that moral machine in there. What compels us to think in moral categories? That's an example of the image of God in man. We may put different things in those categories, but the fact is that we have the machinery to begin with. It appears that no other creature has that machinery and that's what I appeal to, so I don't have to argue for a blanket morality that everyone has. I go one step deeper than that and say even with the difference, and there may be explanations like facts and values for those differences (it's also possible to singe our conscience, if you will), beyond that, let's take the fact of moral machinery. How does one explain that from an evolutionary, physicalist, naturalist perspective?

This is a fact about humankind that we are all well aware of that is very well explained by a Christian view of the world.