People are born with "kinship
detectors" that help us stay away from romantic
entanglements with our siblings that could lead
to evolutionary disaster, a new study
But the system is far from
fail-safe, the scientists found.
The research suggest humans
automatically and unconsciously gauge the
relatedness of people they meet beginning from a
young age. People use at least two separate and
obvious cues to determine whether someone is a
brother or sister: If an individual is younger
than us, we unconsciously observe how much time
they’ve spent with our mothers; if they are
older, we note how long we’ve lived with them.
What we do with that information
was not so obvious until the new study was
On a "moral-wrongness" scale of
19 crimes, people ranked brother-sister sex as
being below child molestation but above other,
relatively minor offenses such as drug dealing
and smoking marijuana. The ranked results, from
most to least immoral, are below:
- Molesting a child
- A man killing his wife
- A woman killing her husband
- Consensual father-daughter sex
- Consensual mother-son sex
- Father-daughter marriage
- Mother-son marriage
- Consensual brother-sister sex
- Brother-sister marriage
- Assault with a weapon
- Robbing a bank
- Selling cocaine
- Breaking and entering
- Smuggling illegal aliens into
- Public drunkenness
- Speeding on the highway
- Smoking marijuana
If through this process we
conclude that an individual is a sibling,
then three things happen: We’re more inclined to
go the extra mile for them; our yuck-factor at
the thought of even making out with them shoots
up; and our aversion to sibling-incest
Inbreeding increases the
likelihood that offspring are born with harmful
mutations that negatively affect health and
For this reason, many species have evolved
ways to spot close relatives and avoid
mating with them.
"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.
Cosmides also points out an
unsettling flip side to using cohabitation as a
sign of relatedness. "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," she said.
The study is detailed in the
Feb. 15 issue of the journal
The findings were based on a
survey of more than 600 participants who were
asked questions about their families, including
whether they had siblings, how long they lived
with their siblings and their siblings’ ages.
They also indicated whether they had witnessed
their mother breast
feeding or otherwise caring for a sibling
from an early age.
Participants then completed
questionnaires to gauge their aversions to
sibling incest and how kindly they felt toward
their siblings. Questions included:
- 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?
- How disgusting or appealing do you find the
thought of brother-sister sex?
In another final questionnaire,
participants ranked brother-sister sex on a "moral
wrongness" scale that included other crimes
such as child
molestation, spousal murder, drug dealing
and smoking marijuana.
Participants were then sorted
into two groups: those who witnessed their
mothers raising and caring for their siblings
from infancy, and those who did not.
Results showed that people who
saw their mother raise their siblings were more
likely to feel kindly toward their siblings,
more likely to find sibling incest morally wrong
and more likely to feel disgusted by thoughts of
sexual acts 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
This makes sense from an
evolutionary perspective, Cosmides said. "We’re
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," she
For participants who did not
grow up seeing their mother raise their
siblings—for example, because their sibling is
much older—their aversion to brother-sister sex,
their level of sibling altruism
and their feelings about the moral wrongness of
incest was correlated with how long they lived
with their siblings: The longer the
co-residence, the stronger their aversion to
sibling incest and their loyalty toward their
"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, Lieberman
said in a telephone interview.
Lieberman likens the unconscious
development of sibling-incest aversion to a
child’s acquisition of language.
"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
The researchers predict that
similar kin detection mechanisms exist in other
social species such as chimps
animals that live in groups where individuals
frequently come into contact with one
In contrast, sea
turtles, which scatter after hatching, would
have little use for kin detectors. "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,"
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, Lieberman said.
"In a zoo," she said, "you might
find all sorts of inbreeding going on."