Human moral instincts are a combination of biological instincts and culturally learned social beliefs. Our biological instincts come from our evolution heritage as a tribal, social species. Culturally learned social beliefs by contrast are created by individual societies and often vary a great deal on what is considered acceptable versus unacceptable behavior.
While each generation is born with the same inherited biological instincts, how those instincts are channeled and expresses can be radically different and change overtime.