History Analysis / BrotherSisterIncest

At the same time, different societies have varying ideas about legality, 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 more complicated. 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. It is critical to note consent is always dubious in the case of incest -- "consenting adults" here can also refer to a parent and child over 18.

There are social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentalIncest. 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 IDontWantToRuinOurFriendship. 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 ObliviousAdoption, 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 SurpriseIncest 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."

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. 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.

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 just don't allow blood siblings to marry, while others also prohibit adoptive or step-siblings from marrying.

!!Social and Ethical Reasons why Sibling Incest is Stigmatized

!!! For a look at the genetics, see '''[[Analysis/IncestIsRelative here]]'''

!! Other
There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentalIncest. 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. 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.

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 just don't allow blood siblings to marry, 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 ObliviousAdoption, 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 SurpriseIncest 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."

There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentChildIncest. 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. 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.

!!! For a look at the genetics, see [[Analysis/IncestIsRelative here]]
!!! For the Westermarck Effect hypothesis, see [[UsefulNotes/WestermarckEffect here]

!!Why Incestuous Relationships are Banned/Taboo and Why it is Rare for Them to Occur Outside of Media

For reference, every relationship is technically incest in the broadest sense, 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 ones, as 3rd cousins are not known to produce mutant offspring. Relationships ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy when graphed]]) show that third cousins only have an "r" rating of .78%, whereas half siblings have an "r" rating of 25%, which is 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% chance of having no copies of the gene and still dying. Legislation in certain areas allows great-grandparents and their great-grandchildren 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.

The UsefulNotes/WestermarckEffect is a hypothesized psychological response of people who are raised together in a family unit from a very early age, which desensitizes them to sexual attraction towards those whom they instinctively consider family members. This is thought to be true even if both people are not related (3rd or more). The most famous case supporting the Westermarck effect was the Israeli collective farms (kibbutzim), in which children were raised communally in peer groups based on age rather than biological relation. One study showed that out of 3,000 marriages that occured within the kibbutz system, only fourteen were between children from the same peer group. These fourteen were seen as the exception that proved the rule, since none of them had been reared together during the first six years of life. This, the thinking goes, is why most people who are in an incestuous relationship seem odd: somehow, their natural instinct was broken, which is usually done through some traumatic experience(s). The hypothesis of Genetic Sexual attraction seeks to explain why close relatives who first meet as adults sometimes experience a powerful sexual attraction to one another. People tend to be attracted to people who are similar to their parents. Genetically it is better if the mate one finds is ''not'' their actual lineal relative, which is why the Westermarck effect is hypothesized to exist. Presumably, if one never bonds with their relative during their formative period, the the Westermarck effect has no chance to take hold and prevent this genetic sexual attraction. On the other hand, a 2009 study by Eran Shor and Dalit Simchai noted that while there were few marriages between kibbutz children from the same peer group, they did find that a significant number felt attracted to their co-rearer peers. They concluded that the kibbutz system provides weak evidence for the existence of the Westermarck effect, and that social taboos and norms are necessary factors to consider. Indeed, people are taught by religious, legal, and medical authorities that sexual relations with one's sibling are sinful, criminal, or a form of mental illness. Even if attraction is felt, it will probably be repressed, especially if the one experiencing it thinks it is one-sided.

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 just don't allow blood siblings to marry, 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 ObliviousAdoption, 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 SurpriseIncest 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--thus avoiding any damage to the gene pool--others who knew that they were siblings would find the idea of them having sex gross "just because."

There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentChildIncest. 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 against parents' right 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. 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.

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 a bit arbitrary. Practically all jurisdictions punish rape or statutory rape of a family member. In cases of two consenting adults, 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 just don't allow blood siblings to marry, 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 ObliviousAdoption, 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 SurpriseIncest 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--thus avoiding any damage to the gene pool--others who knew that they were siblings would find the idea of them having sex gross "just because."

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 a bit arbitrary. Practically all jurisdictions punish rape or statutory rape of a family member. In cases of two consenting adults, 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 just don't allow blood siblings to marry, while others also prohibit adoptive 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 ObliviousAdoption, 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 SurpriseIncest 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--thus avoiding any damage to the gene pool--others who knew that they were siblings would find the idea of them having sex gross "just because."

There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentChildIncest. 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 against parents' right 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. 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 that kind of hardship.

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 a bit arbitrary. Practically all jurisdictions punish rape or statutory rape of a family member. In cases of two consenting adults, it's all over the place. Some countries apply criminal penalties towards any sexual relations between siblings, while others do not criminalize acts between consenting adults and merely prohibit them from marrying. Some do not allow blood siblings to marry, while others also prohibit adoptive 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 ObliviousAdoption, 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 SurpriseIncest 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--thus avoiding any damage to the gene pool--others who knew that they were siblings would find the idea of them having sex gross "just because."

There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentChildIncest. 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 against parents' right 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. 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.

There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentChildIncest. 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. Modern society does not want to permit any exception that might be taken advantage of by abusers, nor would it act against parents' right to make decisions about their children, so it does not 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.

!!Why Incestuous Relationships are Banned and Why it is Rare for Them to Occur Outside of Media

For 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 ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy when graphed]]) show that third cousins only have an "r" rating of .78%, whereas half siblings have an "r" rating of 25%, which is 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% chance of having no copies of the gene and still dying. Legislation in certain areas allows great-grandparents and their great-grandchildren 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 natural programming, as two people who grow up together (usually in the same house) are unlikely to feel sexually attracted to each other because [[UsefulNotes/WestermarckEffect 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 in an incestuous relationship seem odd. Somehow, their natural instinct was broken, which is usually done through some traumatic experience(s). Apart from the Westermarck effect, people are taught by religious, legal, and medical authorities that sexual relations with one's sibling are sinful, criminal, or a form of mental illness. Even if attraction is felt, it will probably be repressed, especially if the one experiencing it thinks it is one-sided.

There are also social reasons for sibling incest to be stigmatized, even when considering a case of NotBloodSiblings. 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 ParentChildIncest. 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. Modern society does not want to permit any exception that might be taken advantage of by abusers, nor would it act against parents' right to make decisions about their children, so it does not 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.

For 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 ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy when graphed]]) show that third cousins only have an "r" rating of .78%, whereas half siblings have an "r" rating of 25%, which is 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% chance of having no copies of the gene and still dying (happened to two of my cousins). Legislation in certain areas allows great-grandparents and their great-grandchildren 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 [[UsefulNotes/WestermarckEffect 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 in an incestuous relationship seem odd. Somehow, their natural instinct was broken, which is usually done through some traumatic experience(s).

For 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 ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy when graphed]]) show that third cousins only have an "r" rating of .78%, whereas half siblings have an "r" rating of 25%, which is 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% chance of having no copies of the gene and still dying (happened to two of my cousins). Legislation in certain areas allows great-grandparents and their great-grandchildren 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 [[WestermarckEffect 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 in an incestuous relationship seem odd. Somehow, their natural instinct was broken, which is usually done through some traumatic experience(s).

For 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 ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy 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).

[[Analysis: Why Incestuous Relationships are Banned and Why it is Rare for Them to Occur Outside of Media]]

For 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 ([[http://en.wikipedia.org/wiki/Coefficient_of_relationship#Human_genealogy 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 done through some traumatic experience.