But what exactly are those differences? And where do they come from?

Though these two questions are interconnected, the first one is much easier to answer than the second. To see how boys and girls play, you just have to watch them do their thing. That’s what Lego did over the course of its research in past decade. For one project, Lego gathered a group of boys and asked them to build a Lego castle together. Separately, they gave the same task to the group of girls. Both groups worked together to build the castle, but once it was assembled, there were stark differences in how the two groups proceeded.

“The boys immediately grabbed the figures and the horses and the catapults and they started having a battle,” McNally said. “The facilitator said, ‘What about the castle?’ And they said, ‘Well, that’s just the backdrop for the battle.’”

The girls, on the other hand, were more focused on the structure—and not too impressed with what they found. “They all looked around inside the castle and they said, ‘Well, there’s nothing inside,’” McNally said. “This idea of interior versus exterior in the orientation of how they would then play with what they built was really interesting. If you think about most of the Lego models that people consider to be meant for boys, there’s not a whole lot going on in there. But [the girls had] this idea of, ‘There’s nothing inside to do.’”

“Both girls and boys were saying they liked building, but there were nuances in what they were looking for,” he added. “We heard girls overwhelming saying we would much rather build environments than single structures. They were really just looking for a lot more detail than we were offering.”

So Lego decided to offer something new. In 2012, the toymaker launched Lego Friends—stylized on boxes with a heart dotting the “i,” and a butterfly hovering nearby—a new line designed for girls. The Friends line includes a pop star’s house, limousine, TV studio, recording studio, dressing room, and tour bus; a cupcake cafe; a giant treehouse; a supermarket; and a hair salon, among other construction sets. (A science lab that was part of the Friends line, which included a pet robot and tiny microscope, is now listed on the Lego site as “retired.”)

From the beginning, Lego Friends sparked a vocal debate. NPR asked, “Why Do Girls Need Special Legos?” A writer for The New York Times lamented Lego’s designs, which she said looked “far more Polly Pocket than Lego Hermione Granger.”

“There were a lot of people at the beginning who said, ‘They’ve dumbed it down, it’s not nearly as complicated [as the original Legos], it’s special for girls because they don’t think girls can build,’” McNally told me. “The reality is, just about piece for piece, there are just as many pieces required to put something together [among Friends sets].”