I have brainstormed to think of possible reasons why the incest taboo exists. None of them stand up to logic and critical analysis.
Possible reason 1. It is rape
Not all rape is incest and not all incest is rape. If a person in an incestual* relationship is coercing or forcing the other person into sexual acts, this is already covered by rape. If a person commits sexual acts with someone under the age of consent, and they happen to be related to them, this is already covered by statutory rape.
Perhaps most incest is rape but this should not be used to blanketly dismiss all incest as rape. As an analogy, perhaps most herbal medicine is not effective but this does not mean that all herbal medicine is automatically ineffective. One should investigate each medicine's effectiveness on its own merits. The same applies to adult relationships. Some may be consenting and others may not.
* The spellchecker thinks this should be "instinctual".
Possible reason 2. Genetic defects in resultant children
I looked for conclusive evidence on rates of birth defects, etc. from incestuous relationships but there are few studies. One that I found said this:
"If the data are censored to exclude physical and mental abnormalities among the male and female parents, and major disparities with respect to young and advanced maternal age, few differences remain in the overall health outcomes recorded for each group."
So this isn't even necessarily true. Even if it is true though, it doesn't explain why procreating in an incestuous relationship is seen as so much more taboo than teenagers or older adults having children. Babies born to teenage mothers and mothers over 35 seem to be more prone to health risks, but these people are not criminalized for having reproductive sex. Teen sexual activity and teen pregnancy are somewhat stigmatized, but here is a thought exercise: How bad is someone made to feel if they have a child as a teen? Rate on a scale of 1 to 5 or something. Now consider how bad society would make them feel if they have an "incest baby". Worse, right? Additionally, people with genetic disorders are not legally prevented to not have children.
And here's a newsflash! NOT EVERY SEXUAL RELATIONSHIP INCLUDES REPRODUCTIVE SEX OR PLANS FOR BIOLOGICAL CHILDREN!!! Are there any laws that say "Okay, you can marry/have sex with your sibling but only if you're both the same gender, or you can prove that you aren't fertile."? I highly doubt it.
So that's reason #2 down the toilet.
Possible reason 3. Evolution favors non-incestual relationships/It's too awkward
Fetish advocate Raven Lightholme has trotted this argument out a couple of times: We evolved to not be attracted to people we grew up around, which was usually our relatives, so that is why people are not attracted to the people they grew up around, and that is why incest is awkward and taboo.
That's an obvious logical fallacy: Argument from evolution. "Evolution favors a certain path, therefore that path is automatically good and others are automatically bad." Let's examine this logic in a few different manifestations:
- Evolution favors non-incestual relationships. Therefore incest is always bad and we should make it illegal and taboo.
- Evolution favors the organism that has the most children. Therefore everyone should have as many children as possible, and anyone who is not constantly pregnant or trying to conceive shall be made a criminal and a pariah.
- Evolution favors agent detection, so we should all believe in gods or conspiracies. Such belief shall be made mandatory by law.
- Evolution favors those who have all senses and body parts fully functional, so it's now illegal to be blind, deaf, paralyzed, missing any body parts, etc.
See how irrational and stupid this argument is?
The other half of Raven's argument is that incestuous relationships are awkward. Sure, why not! Because we've all seen the massive amounts of surveys from people in incestual relationships, revealing that they all are awkward all of the time, and no other relationship is ever awkward! Here are some more awkward things that I guess are also illegal and frowned upon in Raven's universe:
- being called on to answer a question you don't know
- going to a party where you don't know anyone
- talking to extended family members
- explaining sex to your pubescent child
- a homeless person asking you for money
- moving to a new town
- job interviews
- asking someone on a date
- breaking up with a boyfriend/girlfriend
You get the idea.
I sent Raven a message in 2011 asking her to clarify her viewpoint. She has so far not responded.
Possible reason 4. Some people find it gross
This is the worst reason of all. I'll quote William T. Jarvis, Ph.D. from his essay "Why I am Not a Vegetarian": "Whether something is repugnant is highly individual. Hindus who will not eat animal foods readily drink their own urine for the sake of health. And what is repugnant-for example, chores such as changing a baby's diaper or caring for sick people-is not necessarily wrong."
No reasonable person supports banning homosexual relationships on the basis of some people being grossed out by the idea. Why should consensual incestual relationships be treated any different?
Possible reason 5. Against religion
"There appears to be no particular rationale for the subdivision of human populations into opposing forms of marriage preference, and even within the major religions there are quite marked differences in attitude to close kin marriage."
Plus, every religion has things that the members of that religion do which aren't enshrined in law! Jews don't lobby for non-kosher foods to be banned or for mandatory bar mitzvahs for every boy in the world/country! Such laws would never pass.
So yeah it makes absolutely no sense to me that society says all incest is always bad. It's as ridiculous as the notion that all chemicals in all amounts are bad, or that all pharmaceutical companies are evil, notions that no skeptic would support.