Incest

Incest is common among animals because no one is there to tell or show them not to do that or that this often results in crazy or insane or  other physical problems when brothers and sisters or mothers and sons or daughters and fathers mate.

It was once also common among Kings Queens and Emperors in order to keep the money and power in the family. But, as time moved on they found it better to marry their children off to surrounding kingdoms so they wouldn't go to war. So, marrying one's daughter off to the prince who would be king in the next kingdom made more sense than having a daughter marry a son who would be king and have crazy or deformed kids as a direct result and make the next rulers really really nuts for everyone in the kingdom to deal with.

Over time it was learned that genetics are diverse and one needs more diverse genetics to have enough code to be sane and functional. So, it is useful to have children with people not in your same line. As people learned more about this even cousins stopped marrying and having kids as much.

Eventually, it became codified into Marital laws throughout Europe and the United States where siblings or children and parents were not allowed to marry.

It still occurs, however, in people who aren't told what is going on, especially younger girls who aren't told what sex is about around the world. They might just think that being sexual is being affectionate when it is much more than that and can often result in babies if the girl is past girlhood and has begun to menstruate.

So, though it does still happen in a lot of cases it is more likely if an alcoholic or drug addict is involved in this. Because mental and emotional and spiritual boundaries are more likely to be broken when one is drunk or on drugs. And often this can result in permanent psychological wounding when a person grows up and realizes what actually has happened.

So, by educating your children early about what sex is and what it is for you can avoid the harm that incest can bring to a family both psychologically but also through unwanted children that might be born that won't be sane or without physical defects.

Later: My 18 year old daughter was telling me about an episode of Madmen with Don Draper when Don Draper's daughter (I think she is between 8 and 11 years old) goes next door and see someone handsome on TV and starts masturbating because no one told her that isn't polite to do in public. The mother next door gets really upset and brings her back to Betty her mother very upset about the whole thing. This is one of the things that can happen (among many others) if you don't teach your children what is polite to do in public and what is not. However, this incident happened supposedly in 1963 to 1965 which had quite different mores than now.

This is just one of many things that can happen when children are not taught young enough what is acceptable in all situations and what isn't.