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Analysis / Brother–Sister Incest

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For a look at the genetic issues of inbreeding, see here

For the Westermarck Effect hypothesis, see here

Social and Ethical Reasons why Sibling Incest is Discouraged

There are social reasons for sibling incest to be stigmatized, even when considering a case of Not Blood Siblings. It is difficult to consider incest apart from issues of sexual abuse and whether consent is possible, since there may be a difference in age, maturity, or power between two siblings similar to the disparity inherent in Parental Incest. Obviously it would be heinous for someone to rape or sexually molest their sibling, but even if two siblings mutually pursued a relationship with each other, it would be dangerous if they were not of an age to understand the consequences of their actions. There may be a tendency not to permit any exception that might be taken advantage of by abusers, and not to limit the right of parents to make decisions about their children, so perhaps it's not surprising that a society wouldn't recognize such a thing as consensual sex between siblings.

Even if we assume good intentions, romance creates different kinds of tension and conflict compared to the disagreements between family members. When two lovers undergo a nasty breakup they can get out of each other's lives and fall back on friends and family, but if the same were to happen between siblings it could ruin the familial bond between them, perhaps irreparably, like a familial version of I Don't Want to Ruin Our Friendship. Any sibling couple that insisted on staying together would also face extreme hardship from being ostracized by friends, family members, and institutions, to the extent that one would be forced to wonder whether their sibling was really the only lover they could possibly be happy with. Well-meaning family members, social workers, religious figures, etc., besides wanting to uphold social norms in general, might be trying to spare them from having to go through social or legal hardship.

Still another factor is that, although the extent of such expectations may vary from culture to culture, as a rule, family members are expected to support and show affection to one another. This means that, under ordinary circumstances, you may, and even should, show affection or provide support to a family member without arousing the jealousy of your spouse or significant other, jealousy that would be reasonable absent the family relationship. However, in families where incest is practiced — including parental child sex abuse, said spouses and significant others do, in fact, tend to see the perpetrator’s family members, including their own underage children, as romantic rivals. Thus, the incest taboo serves to preserve family bonds from being undermined by jealousy.

At the same time, different societies have different ideas about what kinds of relationships should be illegal, and the choice of which genetic or socialization factors to consider more harmful can be kind of arbitrary. Practically all jurisdictions punish rape or statutory rape of a family member. In cases of two consenting adults, however, it's all over the place. Some countries apply criminal penalties towards any sexual relations between siblings (Iran even has the death penalty as an option), while others such as Japan and Russia do not criminalize acts between consenting adults and merely prohibit them from marrying. Some only prevent blood siblings from marrying, while others also prohibit adoptive or step-siblings from marrying.

Certain thought experiments might turn out very differently depending on the legal jurisdiction and culture. For example, if two unrelated children were raised as siblings in an Oblivious Adoption, then they could concievably fall in love with each other while believing that they were blood siblings. They would angst about this, but if it were suddenly revealed to all that they were not blood related, then they might suddenly find themselves legally able to marry despite the fact that this wouldn't necessarily make the family harmony problems described in the preceding paragraph go away. Conversely, if two long-lost siblings met and fell in love without realizing they were both related, then Surprise Incest could derail their wedding plans or create pressure to break up despite them having never been siblings in any social or psychological sense. Even if they were to use birth control and decide to adopt instead of producing children of their own—or "better" yet, if they were of the same sex, or otherwise incapable of producing a child by incest—others who knew that they were siblings would find the idea of them having sex to be gross "just because."

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