Why We Suck at Judging the Strength of Knots

Despite how frequently we deal with knots—in our shoelaces, in our corded earphones, in our holiday gift-wrapping—they might be giving us more trouble than we realized.

Two researchers from Johns Hopkins University have revealed that people consistently have a surprisingly hard time wrapping their minds around knots. Their findings, detailed in a September 23 study published in the journal Open Mind, suggest that knots may represent a new “blind spot” in our physical reasoning.

The inspiration for the study came from embroidery. One day, Sholei Croom, a PhD student in Chaz Firestone’s lab—both of whom co-wrote the study—flipped her embroidery over to the back of the design and couldn’t make sense of how to handle the tangle of embroidery floss, even though it was her own work. Instead of reacting the way most of us would have (by giving up or reaching for the scissors), she suspected that knots could present a strange gap in intuitive physics: what we expect from the world around us just by looking at things.

“People make predictions all the time about how the physics of the world will play out but something about knots didn’t feel intuitive to me,” Croom said in a university statement. “You don’t need to touch a stack of books to judge its stability. You don’t have to feel a bowling ball to guess how many pins it will knock over. But knots seem to strain our judgement mechanisms in interesting ways.”

The test Croom and Firestone conducted for the study was relatively simple. It involved four similar knots with varying strengths, ranging from one of the strongest (the reef knot) to one of the weakest (the grief knot). The researchers asked participants to look at one pair of knots at a time, and guess which was strongest.

The participants failed spectacularly. They were then presented with videos of each knot slowly rotating, and they failed at this, too. A third iteration of the experiment presented participants with diagrams of the knots’ construction next to each knot—but that didn’t seem to help, either. The few times the participants guessed correctly, they did so for the wrong reasons. The researchers concluded that most people simply cannot differentiate a weak knot from a strong one by sight.

“People are terrible at this,” said Firestone. “Humanity has been using knots for thousands of years. They’re not that complicated—they’re just some string tangled up. Yet you can show people real pictures of knots and ask them for any judgment about how the knot will behave and they have no clue.”

The participants, however, were non-experts, and Croom speculated that individuals with more knot experience—such as sailors or mountaineers—might perform better. Nevertheless, she suggested that it may be harder for people to intuitively understand soft objects like string or rope compared to solid ones.

“We’re just not able to extract a salient sense of a knot’s internal structure by looking at it,” Croom added. “It’s a nice case study into how many open questions still remain in our ability to reason about the environment.”

So the next time you have to show a child how to tie their shoes for the fifteenth time, remember to have a little sympathy—chances are you don’t know your way around knots much better than they do.