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Incest is the activity of having sex with your close relatives. It's opposed by just about everybody[notes 1] — including most liberals/atheists/skeptics/rationalists and fundamentalists — although the reasons vary from: the adverse effects of incestual reproduction, to "God forbade it", to personal disgust.
In the Abrahamic religions, Adam and Eve (the first humans) had offspring who reproduced with each other, namely Cain and his sister. This means that God necessitated the first instance of incest by requiring it of Adam and Eve's children. Abraham also practiced incest with his half sister Sarah (Genesis 20:12), and there would have been a substantial amount of incest going on after the Flood since all of humankind that remained was a single family.
Laws regarding first-cousin marriage around the world
The biological result of too much incest (if safe sex is not practiced) can be the manifestation of normally rare, recessive, genetic traits with unpleasant consequences. After too much reproductive incest, the family tree starts to look more like a ladder. A classic example is the unfavorable traits (such as hemophilia) that became prevalent in the European royal families in modern times. Incest can also confuse familial relationships (imagine "Luke, I am your father...and your uncle"), which is likely the reason it has been banned even in primitive societies that had no idea of the genetic problems of inbreeding.
Ironically, incest is vital to the development of extreme traits in prized domesticated plants and animals. This is called "line breeding", and it is typically done by mating offspring with their parents. Since a few generations of this can lead to problems as described above, then there is usually an "out breeding" to remix the genes a bit.
As many (though not all) undesirable traits are recessive, crossing two different inbred breeds usually produces offspring with a lot of desirable traits. Those traits are however less manifest in subsequent generation. As such a cross is called a "hybrid", that's where the term "hybrid seeds" comes from and that — not some weird Monsanto conspiracy — is the main reason why sometimes it is better to buy new seeds every year, instead of taking them from the last harvest.
Whether or not the same thing that works with plants could work with humans is very much open to debate, but some hereditary diseases have a higher occurrence in some populations and as such children with diverse ancestry have a lower chance of suffering from them. Also, Derek Jeter.
Individuals who practise incest can be referred to by various nouns including inbreeders, incestuals, incesters, incestophiles or incestualists. Occurences between siblings can be referred to as sibcest, between twins as twincest, and that between first cousins where such a relation counts as sufficiently consanguineous, cousincest. Occurrences in fictional accounts involving roleplay is sometimes referred to as fauxcest.
Incest is frequently a form of abuse or pedophilia, as the most common occurrence is between siblings (an older sibling preying on a younger) or between a parent and a child. Both boys and girls can be the victims of this sort of incestual abuse by a family member, and although with boys it is more likely to be male-on-male, both genders are equally likely to experience physical coercion.
 Issues with argument
One problem with most anti-incest arguments is that they tend to focus only on procreative sex, ignoring the likely possibility that many incestuous couples don't want, or even can't have, children (like gay incestuous couples, or those that use birth control). Hence, the genetic argument against marriage between relatives represents a false dichotomy; marriage can be allowed while procreation is restricted.
 Other twists to have in mind
Something to point out are notorious cases in the world where two persons met each other, fell in love, got married, but discovered later on with DNA tests that they are in fact genetically related (most often as siblings who were separated and adopted by different families early in their lives). While this implies they have a higher than average chance of conceiving a biological child with genetic defects, It doesn't erase the times they bonded together, and as a result, the way they view each other, as lovers.
One last idea is that "incest" does not necessarily indicate pedophilia (underage members with an adult) nor rape (one side is not consenting). One should keep that in mind when discussing the idea of incest itself, so as not to confuse or associate it with sexual harassment or violating one's body or lack of Informed Consent.
Incest between close relatives is illegal in most countries. The European Court of Human Rights has decided that individual nations of the European Union may enforce laws against incest.  Sex between adult brother and sister is legal in many countries of the EU and in some states of the USA. 
- "Incest is best, put your sister to the test". -- Anonymous
- "The trouble with incest is that it gets you involved with relatives." -- George S. Kaufman
- "You should make a point of trying every experience once -- except incest and folk-dancing." -- A. Bax, "Farewell My Youth"
- "If you can't keep it in your pants, keep it in the family." -- Anonymous
- "Why go out when you can go down the hall?" -- My friend Jon, but he steals everything so probably anonymous
- "Why go down the hall when you can just go to the top bunk?" -- Jon Bontrager (allegedly)
- "If you ask me a priori the question ‘if a brother and sister love each other and use contraception, is there something absolutely morally wrong about that?’ … I’d have to think about it because I don’t think there’s any absolute condemnation of that fact. If they love each other and care for each other and they go off and it doesn’t affect anything else …" -- Lawrence Krauss.
- "How do you castrate a hillbilly? Kick his sister in the jaw!" -- a well-known joke
 In popular culture
- About half of all redneck jokes (about rednecks, not by them, though possibly that, too), if redneck jokes can be considered to be "culture".
- Played with in the case of Luke Skywalker and Princess Leia, but ultimately averted.
- She's my daughter *slap* She's my sister *slap* She's my sister and my daughter!
- Major plot point in George R.R. Martin's book series A Song of Ice and Fire, and its TV adaptation Game of Thrones.
- Half brother, sister incest plays an important part in the 1996 movie Lone Star.
- That Folger's coffee ad.
- In the T.V. series Rome Octavian has sex with his sister Octavia. Difficult to know if this is based on fact or not.
 See also
- Is Incest Wrong? This tries to rationally discuss/debate the act of incest by consenting adults.
- ↑ Probably except the ones who are doing it.
- ↑ Cain and Abel in Text and Tradition: Jewish and Christian Interpretations of the First Sibling Rivalry, John Byron - 2011, page 27
- ↑ "Incest", Pat McClendon's Social Work
- ↑ German incest couple lose European Court case, BBC News, 12 April 2012
- ↑ Adult incest is legal in very many countries in EU & world & should be legal in Unenlightened Denmark on YouTube