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Is Common Sense Genetic?

Science tries to explain why some people make such stupid choices.

One warm winter morning in 1991, a pilot and her passenger climbed high above Rainbow Lake, Florida, to practice a simulated instrument flight. Forty minutes later, onlookers saw the airplane take an unplanned, fatal dive.

Upon examining the wreckage, the National Transportation Safety Board reported that neither occupant had been wearing a seatbelt, that the right seat was in full recline, and that both occupants were "partially clothed." The cause of accident, the board surmised, was the pilot in command's distraction with "activities not related to the conduct of the flight."

This incident earned one of the first Darwin Awards—annually given to those who "improve our gene pool by removing themselves from it." The premise of the awards, started in 1993 by Stanford researcher Wendy Northcutt, is that common sense is heritable. In other words, we pass it on to our kids. But do we?

A scientific answer hinges on the obvious: What exactly is common sense? Peter Richerson, who studies cultural evolution at UC Davis, thinks common sense includes "the skills to navigate well in your society." A near synonym, he says, could be competence.

But it's broader than that, too. Richerson says common sense is also probably a "compound of several characteristics." It conceivably includes traits such as risk-taking predisposition, learning, reasoning, decision-making ability, impulsivity, and general intelligence.

Genetics influence all these things. In fact the first law of behavior genetics, says Stuart Ritchie, who recently wrote a book on the scientific study of intelligence, is that "all human psychological traits are partly heritable."

For instance, risky behavior—like that of the Darwin Award winner who swung from a rope he attached to a ten-story construction crane and smashed into a building—is partly genetic. An identical and fraternal twin study published in Behavior Genetics suggests that 60 percent of differences in men's desire for new, unusual, and risky experiences ("sensation seeking") is heritable. Research has also shown that the ability to suppress inappropriate or unsafe behaviors ("inhibition") has a genetic component.

Ditto for bad decisions, such as swallowing a live, five-inch-long fish, as one Darwin Awardee did. Some research indicates that people with a certain variation of a heritable enzyme called COMT learn more rapidly from their mistakes and weigh potential rewards better. Experiments have also suggested that particular genes affect how we gamble.

IQ tests appears to be partly heritable, too. Twin, adoption, and DNA studies estimate that genes account for 50 percent of the difference between individual IQ scores. IQ may be particularly pertinent for the Darwin Awards: A study of more than a million Swedish men found that those with lower IQ scores as kids were more likely to get injured or die in accidents later in life, even after controlling for socioeconomic status, education and other factors.

What's complicated is many of the individual traits composing common sense are "polygenic," which means that not just one but tons of genes are responsible for them. For instance, Stanford developmental biologist Gerald Crabtree found that between 2,000 and 5,000 genes influence our intellectual and emotional abilities.

Likewise, Richerson says thousands of genes may contribute to complex characteristics like common sense. There's no single common sense gene. So being related to a dimwit doesn't necessarily condemn you. Even if you have some of their genes, you definitely don't have all of them, unless you're an identical twin. So that's good.

Because so many genes make up a given polygenic trait, however, the precise genetic equation of common sense and other characteristics is unknown. Most individual genes "make such a small contribution that they cannot be detected even in fairly large scale studies," Richerson says. In short: We know common sense is somewhat genetic but not, yet, which genes to blame.

Another way genetics may affect common sense is through our environment. The budding field of epigenetics—how environmental factors influence the way genes are expressed in individuals and their offspring—illuminates the intricacy of this connection.

To take an unconfirmed example: One controversial study found that if researchers shocked mice during exposure to a certain strong scent, their subsequent offspring feared that scent, even though they'd never experienced the shock or the scent previously. This effect seemed to last up to two generations.

Epigenetic effects in humans could be similarly far-reaching. Research by Marlene Oscar Berman, an anatomy and neurobiology professor at Boston University School of Medicine, suggests that environmental factors such as non-intact family, maternal depression, and paternal antisocial behavior can genetically alter a child's brain development and predisposition to certain psychiatric disorders.

Epigenetics could potentially work in a similar way for common sense, says Berman, with traits passed from parent to offspring courtesy of their environment. For example, a study published in Emotion, Space, and Society explains that environmental factors can "change the way DNA is folded," thereby affecting traits such as mood regulation and impulse control. More research is needed on how such changes impact our children.

Wendy Northcutt dedicated her first Darwin Awards book, a compilation of "awe-inspiring error" between 1993 and 2000, to her parents: "The apple doesn't fall far from the tree," she wrote. Seventeen years later, we know a little more what she means.