Inbreeding (reproduction between related individuals) rises the possibility that offspring will have a decreased health and fertility, as we all carry on harmful mutations, but inside a family they are for exactly the same genes.
Thus, inbreeding increases their chance of perpetuation, which would be an evolutionary disaster. That's why most species have developed mechanisms to avoid mating with relatives.
For example, amongst Bedouins in the Sinai peninsula, in whose culture cousins are obliged to marry amongst them, 8 % of the men are deaf, due to genetic mutations. "What this basically is saying is that no matter how attractive your sibling is to other people, it's a deal breaker if it's your sibling," said study team member Leda Cosmides of the University of California at Santa Barbara.
People possess "kinship detectors", making them stay away from their siblings, but it seems that is not innate, thus not infallible.
People automatically and unconsciously assess the relatedness with others from a young age, employing at least two separate cues. If the individual is younger than us, we unconsciously see how long they've spent with our mothers; if they are older, we note how long we've lived with them.
If the brain perceives the other individual as a sibling, then we're more prone to be disgusted even when such thoughts would emerge; and from here, our general rejection to sibling-incest rises. "By the same token, you could have a full sibling but have been reared apart and have none of these cues; you wouldn't in particular have an aversion to sex with them," said Cosmodes.
To see how these clues influence the later behavior towards the siblings, Cosmodes made a survey on more than 600 subjects, asked of course, how long they lived with their siblings and whether they had witnessed their mother breast feeding or otherwise caring for a younger sibling.
A questionnaire checked their aversions to sibling incest and how their fraternal bonds manifested through questions like "How many favors have you done for a sibling in the past year?", "Would you donate a kidney to a sibling if he or she needed it?", "How disgusting or appealing do you find the thought of French-kissing a sibling?" or "How disgusting or appealing do you find the thought of brother-sister sex?"
Another questionnaire asked the subjects to rank brother-sister sex on a "moral wrongness" scale including another 18 crimes.
Subjects were assigned into two groups: those who witnessed their mothers caring for their younger siblings from infancy, and those who did not experience that.
Those from the first category were more likely to feel just fraternal emotions toward their siblings, and to regard sibling incest morally wrong and experience disgust only by the thought of sex with a sibling. "When people see their moms care for their siblings as a newborn, they automatically register that individual as a sibling," explained study team member Debra Lieberman, now at the University of Hawaii in Honolulu. "And that ratchets up this kin detector, causing it to say 'definitely kin.'"
"This makes sense from an evolutionary perspective. We're mammals, one of the species that have a lot of maternal care. And one of the best cues of who your own mother is who takes care of you the most," said Cosmides.
The second group, which usually compassed the younger brothers/sisters, the aversion against sibling incest, their level of sibling altruism and their feelings about the moral wrongness of incest was linked to how long their cohabitation with their siblings was: the longer, the stronger their rejection of sibling incest and their fraternal feelings. "It looks like about 14 to 15 years of co-residence is the time required to reach the [same] levels of altruism and moral wrongness and sexual aversion as people who saw their mothers raise their siblings," said Lieberman. "As a child you hear babbling and noises, and you put together words and language comes online. You're not aware of that happening, but it certainly does," Lieberman said.
Kin detection processes could exist also in other social species such as chimps and dolphins. In species that lack maternal care, like most reptiles, which scatter after hatching, kin detectors could not exist. "If a species is born and flies the coop, never likely to encounter another close genetic relative, then you're very unlikely to see kin detection," said Lieberman. "Animals in captivity, such as in a zoo, often have minimal contact with siblings early on, so kin detection also probably is less prevalent in those cases. In a zoo you might find all sorts of inbreeding going on."
The list below shows how subjects ranked brother-sister sex amongst other crimes.
1. Molesting a child
3. A man killing his wife
4. A woman killing her husband
5. Consensual father-daughter sex
6. Consensual mother-son sex
7. Father-daughter marriage
8. Mother-son marriage
9. Consensual brother-sister sex
10. Brother-sister marriage
11. Assault with a weapon
12. Robbing a bank
13. Selling cocaine
14. Breaking and entering
16. Smuggling illegal aliens into the country
17. Public drunkenness
18. Speeding on the highway
19. Smoking marijuana