10 Times Correlation Was Not Causation

It’s a scientist’s mantra: Correlation does not imply causation. But sometimes wrong feels so right.

1. EAT ENOUGH CHOCOLATE AND YOU'LL WIN A NOBEL.

If you want to boost blood flow to your brain and (potentially) slow cognitive decay, consume flavanols. The plant compounds, found in green tea and cocoa, are great for getting blood into your noggin. That made New York doctor Franz Messerli wonder: Would a nation of bonbon–eaters be more intellectually accomplished than a country that didn’t consume as much cocoa? In a tongue-in-cheek 2012 paper published in The New England Journal of Medicine, he found that countries that ate a lot of chocolate also won the most Nobel Prizes. Messerli published the study with a wink, but some media outlets took the news seriously, failing to see that a confounding variable was at play—wealth. A richer country (like Switzerland, which has 26 Nobel winners) will have more quality scientific research—and well-stocked shelves of chocolate, too.

2. THE NIGHT-LIGHT BIZ IS IN CAHOOTS WITH YOUR OPTHALMOLOGIST.

Nearsightedness has been increasing worldwide for decades. In some Asian countries, up to 90 percent of adults can’t see distant objects clearly, and in 1999, researchers at the Children’s Hospital of Philadelphia thought they’d found the cause: night-lights. The evidence suggested that kids who slept with a light developed myopia later in life. But two groups of researchers argued that the study failed to see the evidence in front of its nose—myopic parents have myopic kids. And myopic parents, who can’t see well in the dark, are more likely to install night-lights in their children’s rooms.

3. BLACK CATS ARE SO UNLUCKY, THEY'LL GIVE YOU ALLERGIES.

Are black cats bad luck for your sinuses? In a January 2000 paper for the Journal of Allergy and Clinical Immunology, researchers found that people with dark-colored cats suffered more allergic reactions than owners of light-colored cats (or no kitty at all). But the correlation appears to have just been a coincidence. Cat allergies are actually caused by a protein called Fel d 1, which is produced in salivary and sebaceous glands. A research team in New Zealand found that cat allergies simply aren’t related to cat color or hair length.

4. FORGET ABOUT APPLES: HEAD LICE KEEPS THE DOCTOR AWAY.

For centuries, natives of the New Hebrides islands considered a head full of lice a sign of good health. “Observation over the centuries had taught them that people in good health usually had lice and sick people very often did not. The observation itself was accurate and sound,” writes Darrell Huff in How to Lie with Statistics. But the correlation didn’t mean lice are the key to good health—it’s the other way around. Healthy people had lice because their body was just the right temperature, a perfect home for bugs. But when people ran a high fever, their flesh became hot, sending the lice scattering. Lice didn’t cause good health—they preyed on it.

5. SERVING BREAKFAST WILL BOOST YOUR CHILD'S REPORT CARD.

We’ve all heard that kids who eat breakfast do better in school. It makes sense; it’s hard to focus on an empty stomach. But despite their best attempts, researchers haven’t been able to pinpoint why breakfast aids learning—if that’s even the case. A 1996 study of Jamaican students found that, in some schools, kids behaved better after they ate breakfast; in other schools, they acted worse. The gap probably had more to do with each school’s resources than with a student’s daily ration of Cocoa Puffs. Students at well-equipped schools behave better regardless of their diet.

6. STORKS AND BABIES ARE INEXTRICABLY INTERTWINED.

Storks do not deliver babies. That bit of German folklore likely originated because the white stork’s migration rituals last nine months. (Plus, Hans Christian Andersen helped popularize the myth in his short story “The Storks.”) But that hasn’t stopped scientists from acknowledging a striking correlation: Between 1970 and 1985, the number of breeding pairs of white storks in Lower Saxony dropped. Over the same period, the birth rate there also fell. Meanwhile, stork numbers increased in Berlin’s suburbs, where doctors delivered more babies. As Robert Matthews writes in Teaching Statistics, “While storks may not deliver babies, unthinking interpretation of correlation ... can certainly deliver unreliable conclusions.”

7. IF YOU'RE A DIEHARD FAN, LOSING THE SUPER BOWL WILL LITERALLY KILL YOU.

Hours after the Seattle Seahawks lost the Super Bowl in 2015, fan Michael Sven Vedvik died. In his obituary, his family blamed the team’s “lousy play call for Mike’s untimely demise.” The joke echoed 2011 research in Clinical Cardiology linking Super Bowl losses to a 15 to 27 percent increase in cardiac deaths in the loser’s hometown. (The Grim Reaper has tried wearing cleats at least once: When the Steelers’ Jerome Bettis fumbled in a 2006 playoff game, a fan watching from a bar became so upset that he had a heart attack. Fortunately, he survived.) Problem is, the studies don’t take non-football variables into account. And the data in one study investigated deaths that occurred two weeks after the game. “I don’t think that everyone who dies within 14 days of the Super Bowl died because of the Super Bowl,” David Prince of Albert Einstein College of Medicine told Live Science.

8. EMPLOYED? THANK YOUR OVERPRICED GROCERY STORE.

In 1958, economist William Phillips published a paper claiming that when unemployment increased, inflation decreased (and vice versa). “That led nations to start thinking of these two variables as trade-offs,” says Rebecca Goldin, professor of math at George Mason University and director of the website Sense About Statistics. “Some would focus on unemployment while others focused on controlling inflation, but they all saw this as a causal trade-off.” Then came the 1970s, when many countries saw both high inflation and high unemployment. Turns out Phillips’s “rule” was just a short-term coincidence. While inflation can affect unemployment for short periods, it can’t fix joblessness over the long run.

9. LONELY PEOPLE KEEP THE SPA INDUSTRY ROLLING IN THE DOUGH.

In Scandinavia, people fight the cold of winter with cozy candlelit social gatherings. Called hygge in Denmark and koselig in Norway, the tradition suggests there’s a connection between physical temperature and the “social warmth” of friends. In 2011, Yale researchers suggested that people may instinctively reach for that connection in the shower. In a study, they found that lonely people were more likely to take long, warm showers and baths. Was it because higher temperatures make them feel less isolated? Well, the methodology left critics cold. The study used a small sample (only 51 undergrads); of those, 90 percent reported bathing or showering less than once a week. Not exactly a trusty sample. In 2014, a different team tried to replicate the results using a larger (and presumably better-smelling) group, and failed.

10. THE POPE SHOULD PRAY FOR THE WELSH RUGBY TEAM... TO LOSE.

According to the 2008 study “Rugby (the Religion of Wales) and its Influence on the Catholic Church,” the Pope is more likely to die when the Welsh rugby team wins the sport’s Grand Slam. The paper—which appeared in the British Medical Journal’s humorous annual Christmas issue—found no connection between the pontiff’s mortality and teams from other countries. Just Wales. We expect a Dan Brown book about this any time now.