Consequences, empathy, sense of community, cooperation, culture, upbringing, homeostasis, and fear of loneliness, all which are actually demonstrably proven to exist (unless we live in the Matrix or some other philosophical habit trail).
For example, it 'might' be 'rational' to conclude that the consequence of raping someone's else's wife may ultimately cause the retaliation of vengeance, leading to the rapist later being murdered by the victim's husband. However, it's purely subjective, in the sense that both the rapist, or the perspective murderer think they are right in doing so. All thoughts, pertaining to applied behaviors, are subjective.
Simple awareness of consequences refrains many from rape or murder. In a sense, this is one minor facet, in the argument of how 'morals' are 'regulated'. Though, it is not a perfect system. But as stated above, many mechanisms contribute accordingly...