Is morality relative?

Is morality relative? Many people have argued that it is. Consider how there have been cultures throughout history that have approved of horrible practices.

But there is an argument that moral differences between cultures are not that deep - even though they seem to be at first.

Imagine that you had to explain to someone how they could do the right thing in every circumstance. One classic answer would be that you should follow the 'Golden Rule'. As Jesus says in Matt 7:12: "Do to others whatever you would like them to do to you. This is the essence of all that is taught in the law and the prophets." This is a pretty good, short summary of how to be a good person.

One interesting aspect of the Golden Rule is that it's considered an important moral principle in so many (all?) cultures, in one form or another. The ancient Egyptians, Greeks, Buddhism, Confucianism, Hinduism, Jainism, Sikhism, Taoism, Islam, and of course Christianity (as well as numerous other religions and cultures) all endorsed the Golden Rule (source).

So if there's such widespread agreement on the Golden Rule between cultures, then it sounds like moral disagreements between cultures can't go that deep. They have to pay attention to something like the Golden Rule (whatever they call it).

You could also say that it's hard to imagine any culture rejecting this principle while continuing to talk in moral terms (although you might find the occasional evildoer who rejects it). Why? Because when you think about it, rejecting the Golden Rule seems like rejecting the concept of 'morality' altogether. Let's say you have someone who doesn't care at all about treating others the way they'd like to be treated. Have they come up with an alternative moral system? Or have they discarded morality? It feels like the latter.

If so, then the Golden Rule is really fundamental to any kind of morality, and therefore every culture shares a fundamental moral principle (even if it doesn't look like it on the surface). Perhaps cultural 'moral differences' are more like differences in application and not fundamental principles.

It should also be noted that this isn't a minor thing that cultures have in common - you can do quite a lot with the Golden Rule. The Bible points out that if we wrong someone we'd rather that they not take revenge on us, because revenge is painful (Lev 19:18). The Bible argues that 'Doing unto others' implies you can't hold grudges, which is taking the principle quite a long way.

So if it's like this then how can you get horrible cultural practices? There are two things to keep in mind here. First of all, people can have dangerous false beliefs and that might lead to evil cultural practices. For example, human sacrifice won't really appease the gods. Secondly, what we think is right can be influenced by our self-interest - surely self-interest can affect the views of groups and even cultures as well. For example, if slavery had a big economic benefit for a society then you would probably find people there rationalise it a lot more than if it had no benefit. Same thing with a patriarchy supporting a patriarchal culture.