It's interesting that all societies, from every culture and era, share several elemental moral beliefs. It is wrong to simply take whatever you want by stealing. Selfishness is far less desirable than self-lessness. Don't cheat your companions. It is better to be honest than a liar. Bravery is lauded, while running from danger is despised. There are more common ethics that can be found like these. While there are always minor differences in moral codes (should a man have one wife or many, is it the parents' job to raise the child or the village's, is it better to fight other people groups or live in peace, etc.) but within each civilization I believe it is safe to say that there are some ground rules of right and wrong that the vast majority of people have followed and still follow.
Generally when a culture begins to deteriorate their moral standards head downhill. Acts of villainy go unpunished because the ones in leadership allow it (often because they are also committing crimes). Lawlessness and a lack of individual self-government become the norm. People attempt to ignore the little voice in their heads that say "that's not right". The first couple offenses are uncomfortable, with the little voice protesting against the act; after that, it gets easier, as repeated wrongdoing callouses or beats the voice into silence.
The little voice goes by several names: the conscience, the voice of reason, the law of nature. Lewis's argument in "Mere Christianity" is that every man and women in their right mind, barring mental incapability or the occasional warped mind (such as a serial killer), has that little voice speaking to them. However... and here's the part that makes people uneasy... no matter how hard someone tries to obey the rules of right and wrong, they inevitably fail. Nobody is perfect. Even if their outward actions appear to be "good", the inward thoughts are selfish, resentful, or condescending, being flawed in some way.
Lewis denies that this little voice can be herd instinct. He gives the example of a man drowning. If you see a man struggling in the water, your "instinct" will most likely be self-preservation and you will want to refrain from jumping in to help him; however, something deeper than that first instinct would most likely prod you to try to save him, even at personal risk. The little voice has the capability of urging us to override our primal instincts.
Moral ethics as a social convention passed down from generation to generation is not the answer either, at least not if you are looking at ethics as merely a human invention. Lewis delves into this issue in great detail. It was a bit tough for me to understand his line of reasoning here, but what I think he was trying to say was this: If you can look at another group/person's concept of morality and say that you believe it follows more or less closely to the ideal of "right", then you are comparing them to some idea of rightness that you have in your head, one that is universal and not based on whatever that group/person "happened to approve".
Of course, with the advent of "personal truth" in our current era, morals are all about what the individual feels is "right" for him/herself, as long as it doesn't infringe on anyone else's personal feelings or beliefs. This sounds gracious, even respectable, on the surface... dig a little deeper, though, and it becomes "what is right for me may be wrong for you, but that's okay because right is only true if you want it to be". And when you start thinking that way it throws all logic out the window; with the dismissal of logic comes the advent of relativism, "might makes right", and ultimately chaos in the social order. This goes all the way back to the founding of Sophism where Protagoras states, "Man is the measure of all things".
In summary, every human has a basic code of morals embedded into their deepest self. We generally try to do right and avoid wrong. But we all break those moral codes in our thoughts and deeds every day. I believe that this means people have an intense desire to be "good". Yet we cannot live up to the standard of "good" we find in our hearts.
What can we do about this?
I think it has already been done for us.