Why would siblings who don't know they're siblings be attracted to each other?

January 12, 2008 3:30 PM

Why would siblings who don't know they're siblings be attracted to each other?

There's been a lot of publicity about the marriage of a twin brother and sister who were adopted and didn't know about each other. They met as adults and felt, according to the article, an "inevitable" attraction.

Drew thinks this particular story was made up by the way, to get a bill passed in the UK parliament.

So, what's going on in the human brain (and why did the CNN writer just put "inevitable" without feeling they have to explain)?

Being attracted to people like ourselves is good? But not if we're too alike? And something about being brought up together hits a kind of kill-switch on being attracted to brothers and sisters? We don't automatically recognise our own genes and rule that person out as a mate?

posted by AmbroseChapel to science & nature (28 answers total)

Folk are naturally attracted to people who look like them. It is no myth or coincedence that married couples often look alike. Deeply repressed incestual urges and total coincedence probably account for the rest of this tragic story. I think they were seperated at or close to birth btw.
posted by fire&wings at 3:33 PM on January 12, 2008

...something about being brought up together hits a kind of kill-switch on being attracted to brothers and sisters?

This is called the Westermarck effect.

As for the larger question, this page on genetic sexual attraction may be interesting, though how much you want to believe Wikipedia on that is up to you.
posted by you're a kitty! at 3:36 PM on January 12, 2008 [1 favorite]

Confirmation bias.
posted by bingo at 3:48 PM on January 12, 2008 [1 favorite]

Narcissism.
posted by meehawl at 3:57 PM on January 12, 2008

No, we don't "automatically detect" our own genes, for attraction or repulsion. Or, if we do, no mechanism for how that might happen has been proved. Genes aren't magic, they don't communicate telepathically with each other. There's no convincing psychological reason why people with similar genes should be more attracted to each other (or repulsed) than anyone else.

The writer is full of bullshit.
posted by OmieWise at 4:02 PM on January 12, 2008

why did the CNN writer just put "inevitable" without feeling they have to explain?

The writer was quoting Lord Alton of Liverpool. I suspect they felt the quotation marks were explanation enough.
posted by thinman at 4:10 PM on January 12, 2008 [1 favorite]

OmieWise - "There's no convincing psychological reason why people with similar genes should be more attracted to each other (or repulsed) than anyone else."

They might look alike.
posted by fire&wings at 4:11 PM on January 12, 2008 [1 favorite]

On the other hand, that reference free Wikipedia article really seems to be on to something:

In The Simpsons, in the episode "The Regina Monologues", Homer Simpson meets his half-sister Abbie- conceived during Grampa's time in Britain during the Second World War- for the first time and seems to display a sexual attraction to her, despite, or maybe because of, the fact that Abbie looks very much like a female version of himself.

posted by OmieWise at 4:11 PM on January 12, 2008

I think that "genetic sexual attraction" thing is largely untested and being promoted by people who, er, have a dog in that particular hunt. The Westermarck effect has been tested. Genetic sexual attraction has books written about it by people who seem - I am not a genetic scientist, but I'm a skeptical jerk - to be very "OOOH, salacious! Also, this backs up things I am personally interested in!"
posted by thehmsbeagle at 4:11 PM on January 12, 2008

PS: I felt my skeptical jerk rise up in the face of that "Twins separated at birth get married ZOMG!" story, personally. The claim that they were conceived due to IVF struck me as deep bullshit. Twins separated at birth already makes my eyebrows go up, twins conceived via IVF and then given up for adoption makes me think it's a lost plot from Days of Our Lives.
posted by thehmsbeagle at 4:13 PM on January 12, 2008

Immediately thought about this:

George Lucas in Love
posted by nosila at 4:20 PM on January 12, 2008

It's a bit late to do much digging around now, but doesn't the Oedipus complex have some basis in reality - i.e. a male is likely to me attracted to a female who exhibits similar characteristics to his mother?

If so, then it's kinda plausible that twins (who, if you believe the pseudoscience, have a bond already) would be attracted even moreso. Or something.
posted by Chunder at 4:22 PM on January 12, 2008

Not only to twins and siblings who are separated at birth have potential attraction, long lost parents reunited with kids they never raised/knew about do too. See the celebrated case of the hedge fund manager who 'married' his daughter.

The longer you are exposed to a face, the more attractive you find it. Since you’ve been exposed to your own face your whole life, you’re going to find it prettier.
posted by Phalene at 4:46 PM on January 12, 2008

I'm a gay male and my mother constantly accuses me of dating men that look like me. but i'm fat and have a beard -- so we all just look alike anyway.
posted by Bear at 5:07 PM on January 12, 2008 [1 favorite]

>The longer you are exposed to a face, the more attractive you find it. Since you’ve been exposed to your own face your whole life, you’re going to find it prettier.

As humans have only been able to reliably, regularly see their own faces in very recent history, I'm going to have to discount that in terms of evolution.
posted by AmbroseChapel at 5:11 PM on January 12, 2008

Genes aren't magic, they don't communicate telepathically with each other.

No, but they do code for the major histocompatibility complex. This in turn influences a person's scent and is at least part of the basis by which dogs are able to scent-track individuals. There is also evidence linking this to mate selection, at least in rodents.
posted by TedW at 5:12 PM on January 12, 2008 [2 favorites]

@Ambrose

Family sharing the same facial muscle structure as you, share the same expressions. Mirroring your fellow humans is a powerful technique for attraction, and family would naturally mirror you. People can't see their body language when they're being copied, but it still works.

As a human being who recently located her long lost family, I can personally attest to being fascinated by the eye spacing of my new relatives. I don't think I was completely aware of all the gestures and expressions I do until I met these people and realized I fell in with their patterns very, very easily.

I don’t think the intent is incest, I think that it’s a side effect of being able to recognize your own genes, which aids their survival.

Personally I found finding my face on my father’s face slightly traumatic, but being around people who miraculously shared so much in common with me was hypnotic, and I spent about a month in a daze sorting out just how I felt about the experience.
posted by Phalene at 5:58 PM on January 12, 2008 [2 favorites]

The longer you are exposed to a face, the more attractive you find it.

Then how does the Westermarck effect fit in? The whole point was that there was a genetic attraction to people from your family who you did not grow up in proximity to - but the ones that look like you but you have known the whole time are not attractive to you. The exposure to the face seems to have exactly the opposite effect, and as Ambrose said above, claiming your own face is the one you have the most exposure to as you're growing up is pretty weak - I'm not even sure it's true now (when you're a kid, your parents and siblings are going to be far more commonly visible to you than mirrors) and it definitely wasn't true in times past, when mirrors were rare commodities.

Still, to say that genes "know" one way or another how like or unlike other genes are is no more or less silly than that they "know" anything. How do we explain attraction to begin with? Why should one person feel compelled to pine for this person and dismiss someone else's pining for them? It's not all rational weighing of shared interests and objective qualities. So much of it seems to be inexplicable that could as well be coded deep into genetic makeup as anything, and expressed through hormones /scents or something. So if a certain level of similarity is positive, then your "type" might seem to match you in a way, and you might be most attracted to people who kinda look like you. It seems to me that people often think couples match, and often think that's good, or cute anyway... And people always joke about being attracted to their cousin who they hadn't met before - and last century, or in royal families, people used to marry their cousins when necessary.
posted by mdn at 6:34 PM on January 12, 2008

Drew thinks this particular story was made up by the way, to get a bill passed in the UK parliament.

By the way, his reasons are that it's extremely unlikely plus they would have known because they'd have the same birthday etc but the point of the legislation was to have open donor adoptions - so they weren't separated at birth; they were separated before conception. I also don't know if that means they were half siblings.
posted by mdn at 6:42 PM on January 12, 2008

Confirmation bias.

You only think it's confirmation bias because you're ignoring the cases that turn out not to be confirmation bias and focusing on the ones that are.

I believe this effect is real and has been studied, but I couldn't immediately find any links for you. As far as I know it's because we tend to be attracted to people who resemble us, as family members often do.
posted by ludwig_van at 7:10 PM on January 12, 2008

Mdn, humans who are briefly exposed to photographs of people, and then given those pictures, plus several more to rate, tend to rate pictures they have seen already as attractive. Extreme familiarity breeds indifference, but a familiar ‘Girl Next Door’ is an attractive stereotype, even with her traditional naive plainness.

If humans are capable of sex after decades of marriage, than the point where you label a person as ‘sibling/child’ must have a limit.

Even without that, repeated family intermarriages are the norm. Small villages and isolated groupings were where we spent most of our specie’s history, and diverse urban environments, where you don’t know everyone and their genealogy, are the anomaly. It’s safe to infer your ancestors had plenty of cousin marriages thanks to availability, ergo the anti-incest rulings must have been much laxer and people okay with having kids with kin.
posted by Phalene at 7:20 PM on January 12, 2008

Well, one would assume that they have the same recorded birth date. How much kismet is that? You meet someone, you are attracted to them, *somehow*, and you start to share personal information. You're adopted, oh my god! So am I! Holy cow! We have the SAME BIRTHDAY! Holy crap! It must be fate! I suppose that this must somehow come into play. You feel this attraction, then to discover that you are both adopted, and born on the very same day, it must feel destined, doesn't it?
posted by msali at 7:27 PM on January 12, 2008

Here's a longish Guardian story about genetic sexual attraction. I'm not sure it offers any definitive explanations, but there are several pretty interesting accounts in it.
posted by cushie at 8:29 PM on January 12, 2008

No, but they do code for the major histocompatibility complex. This in turn influences a person's scent and is at least part of the basis by which dogs are able to scent-track individuals. There is also evidence linking this to mate selection, at least in rodents.

Yes, but if I read your links correctly, this would favor someone genetically different than not similar to you:

It has been suggested that MHC plays a role in the selection of potential mates, via olfaction. MHC genes make molecules that enable the immune system to recognise invaders; generally, the more diverse the MHC genes of the parents, the stronger the immune system of the offspring. It would obviously be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with.

Yamazaki et al. (1976) showed this to be the case for male mice, who show such a preference for females of different MHC. Similar results have been obtained with fish.

Twins are as genetically similar as you can get, so their MHCs should be identical or at least very similar, depending on whether they were identical or fraternal twins.
posted by scalefree at 7:02 AM on January 13, 2008

I recently read speculation that hormonal birth control can "switch" a woman's MHC preference so that she finds men with similar immune systems more attractive. (There is, however, a fair amount of anti-BC propaganda out there, however, so I don't know how strong the speculation really is.)
posted by occhiblu at 8:12 AM on January 13, 2008

Slightly better article from Psychology Today (which means not peer-reviewed, so take it with a grain of salt):

Women generally prefer the smell of men whose MHC gene complements are different from theirs, setting the stage for the best biological match. But Wedekind's T-shirt study revealed one notable exception to this rule: women on the birth-control pill. When the pill users among his subjects sniffed the array of pre-worn T-shirts, they preferred the scent of men whose MHC profiles were similar to theirs—the opposite of their pill-free counterparts.

This dramatic reversal of smell preferences may reflect the pill's mechanism of action: It prevents the ovaries from releasing an egg, fooling the body into thinking it's pregnant. And since pregnancy is such a vulnerable state, it seems to activate a preference for kin, who are genetically similar to us and likely to serve as protectors. "When pregnant rodent females are exposed to strange males, they can spontaneously abort," Herz says. "The same may be true for human females." What's more, some women report a deficit in sex drive when they take the pill, a possible consequence of its pregnancy-mimicking function.

The tendency to favor mates with similar MHC genes could potentially hamper the durability of pill users' relationships in the long term. While Herz shies away from dubbing hormonal birth control "the divorce pill," as a few media outlets have done in response to her theories, she does think the pill jumbles women's smell preferences. "It's like picking your cousins as marriage partners," Herz says. "It constitutes a biological error." As a result, explains Charles Wysocki, a psychobiologist at Florida State University, when such a couple decides to have children and the woman stops taking birth control, she may find herself less attracted to her mate for reasons she doesn't quite understand. "On a subconscious level, her brain is realizing a mistake was made—she married the wrong guy," he says.
posted by occhiblu at 8:16 AM on January 13, 2008 [2 favorites]

Yes, but if I read your links correctly, this would favor someone genetically different than not similar to you...

Yes, but I was merely trying to show a mechanism for genes to communicate, not validate the FPP. Although I do remember seeing somewhere that people are attracted to others who are somewhat genetically similar but not too similar. I can't find a citation for that though, so I may be wrong
posted by TedW at 6:59 PM on January 13, 2008

In case you're still interested.

"...we found a non-significant tendency for women to rate MHC-similar faces as more attractive, suggesting a preference for cues to a self-similar MHC in faces..."

Though how the MHC expresses itself in facial features is beyond me.
posted by artifarce at 7:19 AM on January 31, 2008