Psychologists have long said that men tend to be better than women at spatial tasks, like mentally picturing and manipulating shapes. Hundreds of studies spanning several decades have shown that on average, men score higher on tests asking them to rotate objects in their heads.

This is one of the few persistent and significant differences scientists have discovered between the brains of men and women. The pattern also aligns, perhaps suspiciously, with stereotypes about men being better at reading maps or parking — or putting together flat-pack furniture.

That last cliche might actually be true. Researchers at the University of Tromsø in Norway recently published a study showing that men are better than women at assembling IKEA furniture.

They were inspired by comments a few years ago from Petra Hesser, then the head of IKEA’s Germany division. Hesser claimed that the conventional wisdom was wrong. She argued that women were better than men at putting together the furniture because they are more organized and pay attention to the manual.

Men never look at the instruction leaflet and have the most problems when assembling our furniture because they think they can do it without help,” Hesser said to a German news agency in 2008.  

"A woman will neatly lay out all the screws while a man will throw them in a pile,” Hesser said. “Something always goes missing."

And yet, when the Norwegian researchers sought to test Hesser’s claim, they found that she wasn’t quite right.

The study asked 40 men and 40 women, all university-aged, to put together an IKEA kitchen cart by themselves. Some people got copies of the assembly manual. Others only had a drawing of the final cart.

With the instruction manual, men and women assembled the cart in about the same amount of time on average, and at roughly the same level of quality. Both genders took around 23 minutes to put the cart together, and on average they only made a few small mistakes, like forgetting some screws, for instance.

For those without the step-by-step diagrams, though, the difference between genders was dramatic. Men took around 24 minutes on average, but women took over 28 minutes, a difference of over 20 percent. Had the researchers not cut people off after 30 minutes, the gender disparity might have been even wider. Women were also far more likely to have major problems with their kitchen carts, like missing a shelf or a railing.

Interestingly, the men without instruction manuals seemed to do just about as well as the men with the manuals. Though they were about a minute slower on average and prone to a few more mistakes, these differences were not large enough to be statistically meaningful.

This study suggests that IKEA’s manuals had a big leveling effect. While men could build the kitchen carts with or without the instruction manual, women struggled without diagrams to help them. And even the women with manuals could not overcome the men without manuals.

“In other words, it appears like women need instructions to perform as well as men do without them,” the researchers write.

A lot of the differences between the men and the women could be explained by gender gaps in spatial ability. Before they began, participants all took a classic test of mental rotation ability. They were asked to match pictures of three-dimensional objects drawn from various angles. The test results showed stark gender differences, in line with many previous studies from other researchers. In general, psychologists find that over 80 percent of men are better than the average woman at imagining how these shapes look when rotated.

It’s obvious why the ability to mentally picture shapes is important to a task like furniture assembly. Given a pile of slats and screws, it requires a lot of visualization to figure out how they can fit together into a three-dimensional shape. Even with instructions, some mental agility is required. IKEA manuals have no written instructions, so people have to be able to follow the diagrams as the figures rotate. One of the chief frustrations of IKEA furniture is that moment when you realize you have to tear apart everything because you installed a panel backward.

Differences in spatial ability have also been used in attempts to explain how women are so underrepresented in the sciences. For instance, engineers have to think about how their robots are assembled, and chemists have to think about how molecules lock together. Psychologists find that people with better spatial reasoning scores in high school are more likely to go into STEM careers. 

But the male advantage in this realm only shows up in the overall averages, which obscure tremendous diversity between individuals. The spread of individual abilities is large, meaning that there are many women are much better than men at spatial tasks. Another way to put it: men’s brains are more different from each other than they are, on average, from women’s brains. Psychologists have been saying this for a long time, and the fact was underscored by a recent study using MRI scans.

The relationship between the gender gap in spatial ability and the gender gap in STEM employment is still hotly debated. Does one imbalance cause the other? Or are both patterns the result of cultural factors that push boys but not girls toward careers in math and science?

Many believe that the differences in spatial processing ability are largely the result of upbringing. In one famous study, researchers administered a visual puzzle to people from two genetically similar but culturally distinct tribes in Northeast India. In the tribe where men ruled, the women tended to perform slower than the men. In the tribe where women ruled, there were no gender differences on average.

Childhood experience with Legos has been shown to predict performance on these sorts of tests, and here the Norwegian researchers found the same pattern. Participants who said they played with Legos as a kid tended to do better on the spatial abilities test and were also better at assembling the furniture.

Strikingly, there is research suggesting that these gender differences are not only malleable, but also fleeting. When women are asked to imagine themselves as men, they perform just about as well as men on a mental rotation test. Women also improve their scores dramatically if researchers tell them that women tend to beat men on this test. Some psychologists argue that confidence plays an important role in explaining the gender gap on mental rotation tasks — men being more sure of themselves and more likely to guess.

The Norwegian researchers emphasize that their IKEA experiment is narrow. They only tested one kind of furniture, and their subjects were Norwegian twenty-somethings. And even though there were some significant differences on average, there was also significant overlap between men and women.

Most importantly, they leave open the question of why these differences show up. Perhaps men were better at assembling IKEA furniture because we expect men to be better at assembling IKEA furniture, and because boys are raised to play with toys that emphasize these kinds of skills. If previous research is to be believed, this may be one example of a self-fulfilling stereotype. The absence of women in STEM careers may be another.