Back in the first grade, you were handed a sheet of paper printed with a solid black line. Aside from evenly spaced, horizontal hash marks, the line was blank. The above instructions likely read: Put these numbers in the correct order. Zero went all the way to the left, and bigger values to the right—right?
This ascending order, though painfully intuitive now, is the subject of a hot nature versus nurture debate: Were you really taught the number line in school or are we all born knowing it? An answer to this question is now faintly closer thanks to some humbling results published in the journal Science.
It turns out domestic chickens may use this number line, too.
Wait, chickens can read…and count? Well, not exactly. But scientists at the University of Padova in Italy imitated counting by training a brood of newborn chicks to find food behind a panel. At first, the chicks were taught to find food by walking behind a panel illustrated with 5 squares.
Then, during testing, the chicks were released in front of two identical panels—one to the chick’s left, the other to the right. Each time, the panel had a different number of squares. The researchers were watching to see which panel the chicks looked behind for food. When the panels had just 2 squares, chicks approached the one to the left 70 percent of the time. But when there were 8 squares on each, chicks walked toward the right panel 70 percent of the time.
Basically, when the number was less than five, chicks went left. When the number was higher than five, they went right. This is pretty convincing evidence that the baby birds associate increasing value with a rightward direction: “especially because all of the chicks underwent both tests,” says Rosa Rugani, the study’s lead author.
If these chicks represent birds as a whole, it means that humans aren’t the only animals with left-to-right number lines in mind. It means that this line probably originated before ancestors of humans and birds parted evolutionary ways— millions of years ago.
This also suggests that humans are born knowing the order of numbers, supporting the idea that “culture is not crucial for the mental number line,” says Rugani. In other words, first grade may not be the first time you see numbers scaling left to right. This tendency may be embedded deep into your shared history with birds.
But psychologists like Samuel Shaki from Ariel University in Israel believe that our mental number lines are largely a product of our upbringing and education— making the fact that the study’s three-day-old chicks also use the method slightly puzzling.
So the notorious nature or nurture dispute endures: Does our human (or avian) sense of numbers exist at birth or do our parents and teachers impart the ascending values upon us? If you’re hunting for a clear-cut conclusion, sorry—it is undoubtedly a messy mixture of the two. “This is indeed a debated issue,” says Rugani, but “our study may have possibly contributed to disentangling it.”
Editor's Note: Since we published this story, other scientists have expressed concerns about possible biases existing in the study's methodology. Those critiques are available here and here. The study has not been retracted, and the authors have responded to those concerns here.