Week 18: Distinguish correlation from causation

bunnies, bunnies, it must be bunnies. Or correlation. |   Mary Toft and Her Extraordinary Delivery of Rabbits.

bunnies, bunnies, it must be bunnies. Or correlation. | Mary Toft and Her Extraordinary Delivery of Rabbits.

Food-based pregnancy calendars aren’t all bad. They’re very useful if you want to develop fetus-related puns. If your last name happens to be Pappas, for example, “Pappasito” is an excellent nickname for your fetus. And in Week 18 you can switch it to “Pappa-sweet-o,” because this week your child’s the size of a sweet potato.

This also means that, if you eat an order of fries in Week 18, you’ve eaten your fetus’ weight in food. Another benefit of the food-based pregnancy calendars is the weekly reminder that fetuses don’t need that many calories.

Our pop culture references are full of pregnant women chowing down on comically large plates of food. But that has not always been the view. In the 1930s, women were recommended to restrict their diets during pregnancy, because weight gain was thought to lead to pre-eclampsia. 

Statistician Bethany King of Graph Paper Diaries uses doctors’ dieting advice to pregnant women as an example of mistaking correlation for causation. In short, when two variables are related, we might mistakenly assume that one of them caused the other. In the case of pre-eclampsia, doctors assumed the weight gain caused the pre-eclampsia, when it was actually the other way around. That mistaken conclusion led to dangerous complications for mothers and babies. 

But before we get too comfortable rebuking last century’s doctors and their backwards thinking...

Some elements of pregnancy are linked by causality. Sex causes pregnancy. Pregnancy causes weight gain. Labor causes delivery. Delivery creates parents.

Many elements of pregnancy only appear to be causally linked. Pregnant women who consume caffeine report more miscarriages, but that doesn’t necessarily mean that the caffeine caused the miscarriages. The miscarriage rate has also been linked to the flu shot, but using news reports about this link to guide your medical decisions could lead to life-threatening illness for you or your baby. Sure, it’s possible that having sex on a full moon in February created a female fetus. It’s more likely that having sex led to a fetus, which had a just south of 50 percent chance of being female.

Correlation may not mean causation, but “it does waggle its eyebrows suggestively and gesture furtively while mouthing ‘look over there.’” Humans are very good at seeing patterns where no patterns exist. We view random unrelated events, like the bread we ate yesterday and our illness this morning, and determine that bread made us sick, even when that bread and our illness are no more related than spelling bee word length and deaths from venomous spiders. We are especially terrible at this when it comes to kids. Maybe that’s because we so desperately want to feel like we’re in control, or at least that we’ve become masters over the chaos.

As with so many of the research lessons you’re learning while expecting, understanding the difference between causation and correlation can make you a more reasonable, less terrified, more adventurous, and ultimately more fulfilled parent. Lots of bad things are going to happen to your baby, and you’re going to hunt like a truffle pig for the root causes. Your baby spit up after feeding? It must be the hot peppers or dark beer you drank. Your baby spikes a fever of 103? It must be because you let your in-laws touch her. Your toddler has a febrile seizure after his first flight? He’s not going on another plane until at least high school. This is no way to live, because hot peppers, in-laws, and airplanes are all good things worth exposing our kids to. 

Want to get avoid leaping to causation? It helps to start with crazy examples, like ice cream sales and the murder rate:

Yes, ice cream sales and murders spike during the summer. You could conclude that ice cream sales are driving people to commit murder, or you could ask what other variables ice cream sales and murder have in common. [Turns out, hot weather drives both.] 

Speaking of ice cream...there's an impressive array of pickles and ice cream merchandise available on Etsy that suggests you or your pregnant partner will soon be demanding that particular blend of salty and savory. If you're the one traveling for those items in the middle of the night, you can blame I Love Lucy