Why We’re Bad at Math and Have Diabetes

April 5th, 2015

Illustration by Mauricio Antón.  © 2008 Public Library of Science

Illustration by Mauricio Antón. © 2008 Public Library of Science


We’ve been hunter-gatherers for 99% of our history. We used to walk between five and ten miles a day, consume five pounds of sugar each year, and scour all day for sources of fat. We craved calorie-rich foods and our bodies figured out how to store energy to survive frequent bouts of famine.


We now live in a materially different environment. The average American now walks about one-third as far, consumes 25 times more sugar, and has ample access to caloric and fatty foods.

Our bodies are wired for famine, but we now live in abundance. As a result, we overeat and struggle to metabolize the excess sugar and fat, leading to obesity and type 2 diabetes. These chronic conditions are mismatch problems partially caused by a gap between our environment and our biology. A mismatch problem occurs when the traits that helped us thrive in our evolutionary past become inadequate in our new environment.


Mismatch problems also exist in the brain. Just as our endocrine system didn’t evolve to handle a high sugar intake, our minds didn’t specifically evolve to create art, philosophize, or make complex decisions.


Consider the Linda problem made famous by Amos Tversky and Daniel Kahneman in 1983 [PDF]:


Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations.


Which is more probable?
A. Linda is a bank teller.
B. Linda is a bank teller and is active in the feminist movement.


If you’re like 85% of people, you’ll choose B because the description of Linda makes her sound like a feminist bank teller. However, basic probability reminds us that there must be more bank tellers than feminist bank tellers, as the latter is a subset of the former, so A must be more probable. This error is known as the conjunction fallacy.

Just as we can’t blame our bodies for struggling to keep our blood sugar stable after eating processed foods, we can’t fault our minds for making mistakes when asked to interpret abstract conjunctions. It’s unlikely that ancient humans needed to ruminate on conjunction problems to excel in the age of the woolly mammoth, so it’s no surprise we make these mistakes. In this way, being bad at abstract probabilities is like having diabetes — these simply aren’t problems that our evolutionary ancestors needed to handle.


How can we solve mismatch problems? We often attempt to enhance our capabilities through education. Unfortunately, the food pyramid is no match for biology programmed to devour the bulk bag of candy that awaits us in the kitchen.

Instead of trying to band-aid our biology, we should focus on shaping our local environment.  We can craft our surroundings, which we often control, to promote the desired behavior. If we don’t buy bulk bags of candy, we won’t need to exhaust willpower to fight the inevitable temptation to overindulge.


The same technique applies to the Linda problem. By reshaping the problem with natural fractions that represent our experience of the world, fewer than 15% of people violate the conjunction rule (Hertwig & Gigerenzer, 1999 [PDF]):


In an opinion poll, the 200 women selected to participate have the following features in common: They are, on average, 30 years old, single, and very bright. They majored in philosophy. As students, they were deeply concerned with issues of discrimination and social justice and also participated in anti-nuclear demonstrations.


Please estimate the frequency of the following events.
How many of the 200 women are bank tellers? ____ of 200
How many of the 200 women are bank tellers and are active in the feminist movement? ____ of 200

Humans have been evolving for about two million years, but the demands of today are a substantial shift from our past. Our diet has changed dramatically in the last few thousand years, and we started using probabilities only a few hundred years ago. Our genetics simply haven’t had time to catch up.


In the meantime, we are simply caught in a state of disequilibrium between our genes and our environment.  We shouldn’t hold ourselves to unreasonable normative standards in light of our evolutionary past. While we can’t yet easily change our genes, we can architect our environment to promote the behavior we desire.