Our children watch and copy everything they see and hear. If their parents, or grandparents are unkind to others or if they lie, steal or cheat, they have no ethics. A child sees this and think it to be a good way to live. You start learning ethics before you even begin school, and it moves on into your adulthood.
By and large, most of our ethical beliefs are learned. We are taught both by family and government what we can and cannot do. But even a small child seems to understand when something they do is not right, so I must also conclude that somewhere along our evolutionary path, some absolute basics were learned that were hard wired into our brains. In case this seems odd, just take a look at the wildlife. Every pack or group displays behavior that says they will not accept certain types of behavior. So either the wolves and big cats are just as smart as us, or a certain amount of behavior code is already in our brains.
I'd like to believe that there is something inherent in most human beings that aren't raised by wolves that would lead them to follow some sort of ethical compass. However, I truly believe that it's more likely that ethics are learned first from one's family or whomever one grows up around.
I do not believe that people are simply born knowing what ethical behavior is, neither does it come naturally to them. I believe most ethical people are taught good ethics by their parents when they are growing up. On the other hand, most unethical people probably were lacking being taught to be ethical growing up.
I think that ethics are learned. The values that children are taught as they are growing up contribute to this. It is not like they are born one day and know how to be kind, respectful, ethical, etc. They see the way that people behave around them and this influences the way in which they behave themselves.
While it is true that the precise ethical values are taught, I'll side with natural just for the fact that many, if not all, human traits are heritable and this includes ethics. Moral values have probably evolved in humans so that they could live in groups peacefully which was good for the survival of the individual.
I was adopted at birth into a conservative family, I was never told until I was in my 20's and by this time I had 2 most amazing children of my own. I never felt a part of anyone when I was growing up and that is ok, because I wasn't, but the amazing thing was I felt things they did not. I watched, heard and saw their prejudices, social banding, their deliberate blindedness, and many more characteristics of what I felt was un ethical. All my so called siblings seemed to just "go with it". When I met my adopted family in my late 20s I felt right at home and my ethics and opnions on humanity could finally speak and be in a crowd of the same ethics and opinions. Hello, my name is Andrea and I was born in 1968. In 1993 I was reborn as myself upon meeting my natural family. I now enjoy my genuine genetic ethics and my adopted family continue to have theirs.