This recent story went wide: British fraternal twins who were adopted separately at birth later married without realizing they were brother and sister.
Why does this make us so instantly and overtly squeamish?
Lord David Alton of Liverpool — a member of Britain's House of Lords — discussed the couple's case during a government session on in-vitro fertilization as he pushed for identity rights of children conceived by the technique.
On his Web site, Alton noted that a similar adopted brother-sister marriage was recently avoided through detailed identity records.
Incest is considered taboo in nearly every human culture around the world, researchers have found. Yet as the 21st century waxes, questions about the behavior remain unanswered.
Where does our aversion to incest come from — genetics or society — and what's so bad about it, anyway?
Scientists think Earth's earliest life emerged about 3.8 billion years ago and slowly evolved into the diversity of organisms seen today.
Until roughly 1.2 billion years ago, however, sex didn't exist.
Nathaniel Wheelwright, an evolutionary biologist at Bowdoin College in Maine, said asexual reproduction was the first type of reproduction to evolve.
In its most basic form, called parthenogenesis, it involves one-celled organisms such as bacteria dividing in two. But more complex creatures do it, too.
"Asexual reproduction is [like] the ultimate in incest because you're breeding with yourself," Wheelwright told LiveScience.
"You can still see species asexually reproducing, or cloning themselves, in situations where there is no advantage to [sex]," he said, "and you can see species that commit incest where there is no penalty to inbreeding."
Aside from microbes, most of which reproduce asexually, Wheelwright said mountaintops, small islands and other isolated habitats are places where today's incestuous reproducers are most commonly found.
"If your relatives are the only game in town, you don't have much of a choice," he said.
But Wheelwright explained that sexual reproduction — the current reproductive norm among plants and animals — gives creatures a leg-up in life.
"Sex results in ... diverse offspring and maintains a diversity of genes," he said.
It's like nature's way of avoiding putting all its eggs in one basket: Where one copy of a gene may spell doom for one organism, a different version spread through sex in another creature may help it survive.
"People who domesticated plants and animals were likely the first to figure this out," Wheelwright said. "When they inbred, they got lower birth weights, increased embryo death and decreased fertility."
Still, genetic diversity is at times less important than other advantages, such as better guarding of offspring in some African fish that inbreed.
On the whole, however, the risk of incest in plants and animals generally outweighs any of its benefits.
The problem with incest is that it can keep so-called "bad" genes in the gene pool and compound their effects, said Debra Lieberman, an evolutionary psychologist at the University of Hawaii.
"Close genetic relatives run the risk of having offspring that have a reduced chance of surviving," Lieberman said.
To understand the dangers of incest in humans, she explained, one needs to know that DNA — the blueprint of life — is divvied up into two sets of 23 chromosomes for a total of 46 in the average human being.
One set of 23 comes from the father, while the other comes from the mother.
While Lieberman cautioned it's never plain when it comes to genetics, she offered a simplified example to illustrate the risks associated with incest.
"Let's say you get a bad gene, which scientists call deleterious, from your mom. But your dad's copy of the same gene functions normally," Lieberman said. "The good version acts like a backup, effectively preventing disease the bad gene might have caused."
But having a kid with your sibling, she explained, drastically increases the chances of getting two copies of the deleterious gene as compared to reproducing with someone outside of your family.
"Each of you would have a copy of that bad gene, so there's a good chance your kid won't have a normal copy to work with," she said.
Multiply that by any other deleterious genes sprinkled among an estimated 50,000 active genes in humans, she explained, and there are bound to be some life-shortening problems.
Because so-called higher organisms such as humans are susceptible to life-shortening genetic combinations, Lieberman thinks nature has weeded out incestuous behavior over time through natural selection.
Humans and other animals, she said, likely evolved ways to detect and avoid mating with their close relatives.
"We don't have DNA goggles to detect our relatives, but I think we've evolved cues that help us do so," Lieberman said, including face recognition and even scent.
But Lieberman thinks the strongest cue humans have is growing up with a sibling under the same roof.
"People refer to this as the Westermarck Effect, which essentially says children who co-reside are much less likely to breed with each other when they reach adulthood," she said.
Even unrelated children who grow up together exhibit avoidance toward inbreeding, she said.
"The Kibbutz communities in Israel are a good example," she said.
Only weeks after birth, mothers give their kids to a "children's society" staffed by trained caregivers.
Lieberman said people raised in the same community are much less likely to marry each other than someone from a neighboring area.
Another example Lieberman noted are 19th-century records of arranged Taiwanese "minor" marriages, where parents would arrange a marriage for their daughter by handing her over to the future groom's household shortly after birth.
"Compared to 'major' marriage arrangements, where a couple meets just before the wedding, minor couples had fewer kids," she told LiveScience. "Minor couples frequently refused to consummate their marriage, so the fathers would stand outside their door until they did."
Lieberman thinks minor couples had such trouble because they grew up with one another, "activating the genetic cues that screamed, 'Avoid mating with this person,'" she said. "Those cues probably didn't get activated with the brother-sister couple who married. They didn't grow up together."
Although no genes for incest avoidance cues have been pinpointed yet, Lieberman thinks they will eventually be tracked down.
"It would be wonderful to isolate those genes," she said. "I think we will some day, but we need to know if there are other cues used to avoid mating with a relative."
But how does Lieberman explain incestuous behavior both in captive and wild animals, such as juvenile male chimps who attempt sex with their mothers?
"These systems aren't foolproof," she said. "Sometimes the [female chimp mother] lets male offspring mount her if she's frightened and wants to calm down. But most of the time, females squawk and reject the attempts."
David Spain, an emeritus University of Washington anthropologist who has followed incest research since 1968, said incest "defeats the whole point of sex" — mixing up the gene pool — and is ultimately why the behavior is astonishingly rare among first relatives.
"Cousin marriages don't have as much in the way of deleterious effects, so we see those partnerships more often," Spain said. "Evolution weeds out the things that don't work."
Better birth certificates?
Spain thinks the now-unmarried twins, whose identities and annulment details have been concealed, would be fascinating to interview.
"This is definitely a one-in-a-million type thing. The psychoanalyst side of me definitely wants to know what was going through their minds after they discovered they were brother and sister," Spain said, noting that such an analysis might offer important scientific clues about incest.
Other than that, he said, the couple's story simply excites human aversion to incest.
"Just look to popular culture to understand why," he said. "It's sort of like a 'Star Wars' story that ends up with Luke Skywalker and Princess Leia marrying each other."
Yet Dan Boucher, a spokesperson for Lord Alton, said the couple's tale might repeat itself as more people choose to conceive their children through sperm donors.
"A donor can be used to conceive up to 10 children," Boucher said, and according to Alton's Web site up to 25 children have been conceived from a single donor. "That greatly increases the chances of something like this happening again."
Offering two birth certificates to IVF children, he said, could help: One "long" version would indicate the genetic father as well as the mother, while a "short" version without such details could be used to maintain the person's privacy.
"I'm hoping this will become a law by the summer," Boucher said.
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