A Jumble of Genes

A Jumble of Genes
Why Humans are Incest Repellent
by Allison MacLachlan

Several winters ago in Britain, a man and a woman fell in love and were married. Their romance was just like any other boy-meets-girl fairytale, minus one serious roadblock: in December 2007, the House of Lords ruled to annul this couple’s union1. As it turns out, the two were related—and not as distant cousins. The man and woman were twins.
BBC News reported the man and woman didn’t realize their genetic relationship until after they were married. The pair of fraternal twins had been separated at birth1. When they met by chance later in life they felt inevitably drawn to one another, a feeling they came to know as love.
Despite the traditional romantic elements of this anecdote—forbidden attraction and the prescient hand of fate—it doesn’t have the universal appeal of a Romeo-and-Juliet, Lancelot-and-Guinevere love story.
In fact, it irks us. There’s something tragic and repulsive about romantic love developing between family members. But fundamentally, why are humans so averse to inter-familial romance?
Pam Hodgkins, CEO of the charity Adults Affected by Adoption, explained for BBC’s news report that “We have a resistance, a very strong incest taboo where we are aware that someone is a biological relative.”1 In nearly every corner of the globe, humans view incest—defined as sexual activity between closely related persons—as at least objectionable and at worst, illegal.
Our deep-seated aversion to incest stems from our biology. Beings that reproduce sexually, from fish to ferrets and hamsters to humans, rely on sex to create variation among their offspring.
A male and female each contribute a half set of chromosomes to the production of a new embryo. For example, in humans, where 46 chromosomes make up a complete individual, the mother and father must each contribute 23.
Along these chromosomes, several genes sit quietly like keynote speakers at a conference. They are the VIPs, the ultimate authorities who will control what happens. They are responsible for disseminating important information to the crowd—in the body’s case, this is a crowd of cells and molecules, rather than CEOs and managers.
The way we function as effective humans largely boils down to our genes. If we inherit some faulty genes as a result of random mutation—and we often do— it’s usually not an issue because, like kids on a field trip in primary school, genes travel in pairs. It’s as if a mother’s genetic contribution holds hands with a father’s contribution. If one member of the pair has a mutation at a specific locus, the other likely does not—given the breadth of the human genome. This is why variation is important: it reduces the odds that congenital birth defects will happen.
In the case of inbreeding, where male and female partners both come from the same familial genetic pool themselves, similar genes become concentrated. As a result, there’s a far higher risk of birth defects in their offspring2.
This becomes a serious problem if you look a few generations down the line in a hypothetical community of inbreeders. With close to 50,000 active genes in any given human’s body, there are countless chances for inopportune pairings to happen—especially when mutated versions of genes keep being traded among the same population2.
Like almost anything else in biology, incest is victim to natural selection. Darwin’s theory of evolution suggests we’re all on a mission to live out our genetic legacy by producing healthy offspring who survive to reproductive age themselves. If our biological destiny is to propel our genes indefinitely into the future, the least advantageous decision would be to put our offspring into life-compromising danger before they’re even born: in essence, to engage in incest.
So, for the benefit of our own natural selection, it’s adaptive to consider incest something we’d never want to be involved in.
But how do we know who to avoid? In a world where competition for mates and genetic legacy is said to be the underlying motivation for virtually all we do as humans, there must be some mechanism to help us recognize who it’s appropriate to mate with—genetically speaking.
One proposal, called the Westermarck effect, suggests children who live in close domestic proximity during their early years lose any sense of sexual interest in one another1. Since children in this situation are often siblings, who naturally share a large percentage of genes, it’s a logical adaptation.
The Westermarck effect is clearest in Israel’s “Kibbutz” communities, where communal caregivers typically raise neighbourhood children in peer groups defined by age, rather than nuclear family units1. One study that followed a cohort of these children into later life found that, out of 3,000 marriages, not a single one took place between kids raised together in the same peer group between birth and age six. And only 14 of the 3,000 marriages were between children who had been members of the same peer groups at any point in childhood1.
The Westermarck effect suggests the genetic health of our offspring depends on who we spend time with when we’re young. For genetically related children brought up separately, it’s easier to slip into sexual attraction as adults.
In a new age of reproductive technology, it’s possible incest could happen purely by accident. Procedures like sperm donation and in vitro fertilization experiment with bringing sexual reproduction out of the body and into test tubes. It becomes harder to recognize your relative’s chromosomes when they’re looking at you not from his face, but from a Petri dish.
So long as we don’t have sensors to alert us when we’re falling for someone too close to our genetic quotient, we should be privy to the potential risks of reproductive medicine and the modern family. Some relationships need to remain platonic in order to keep our genetic legacy on the playing field—but thanks to biology, we don’t even need to be reminded.
1 BBC News. 11 January 2008. “Parted-at-Birth Twins Married.” BBC News [Online]. Available: http://news.bbc.co.uk/2/hi/uk_news/7182817.stm.
2 Mosher, Dave. 16 January 2008. “Incest Not So Taboo in Nature.” Live Science [Online]. Available: http://www.livescience.com/health/080116-incest-science.html.