Why Incestuous Relationships are Banned and Why it is Rare for Them to Occur Outside of MediaFor reference, every relationship is technically incest since everyone is at least your 16th cousin. However, we use the term incest for people who are at least 3rd cousins. In reality, marrying a 3rd cousin is taboo because of social reasons and not genetic as 3rd cousins are not known to produce mutant offspring. Relationships (when graphed) show that third cousins only have an "r" rating of .78% where as half siblings have an "r" rating of 25% which more than 32 times greater. Even if two people are greater than 3rd cousins away, they can still be both missing a copy of one of their genes which isn't a problem since they have a copy. However, the offspring has a 25% of having no copies of the gene and still dying (happened to two of my cousins). Legislation in certain areas allows great grand parents and their great grand children to marry (which is on the same level as 3rd cousins), but in the USA any relationship of 3rd cousins or more related is illegal. All this means is that in media, if a character says "well, they're only half-siblings" it doesn't matter as that relationship still has an "r" rating of 25% which is dangerous to potential offspring. Even first cousins once removed, still has an "r" rating of 6.25%. Lastly, since identical twins are basically clones, they are counted as the same person (genetically) and the offspring they have (with two different people) are still technically half-siblings. If they have a kid with the same person, then the offspring can be considered full offspring. Most incestuous relationships are prevented by nature as two people who grow up together (usually in the same house) are unlikely to engage in sexual activity with each other because their instincts tell them it's wrong. This is true even if both people are not related (3rd or more). That's why most people who are an incestuous relationship seem odd. Somehow their natural instinct was broken which is usually done through some traumatic experience(s).