But it has nothing to do with parents favouring their daughters, says Milligan. “Overwhelmingly, in North America there is a preference for having a child of each sex … which left a bit of a mystery to where this boy-girl achievement gap was coming from.”

The theory he now leans toward is that it’s simply easier and more productive to spend learning time with girls, who are more likely to sit still and perhaps process information more efficiently. “So even if I spend equal time, the time with the girl might be more productive — we don’t have direct evidence of this but the evidence pushes us in this direction,” says Milligan.

From their research, which found similar results across the U.S., the U.K. and Canada, they estimate that spending equal learning time with boys and girls could reduce the Canadian kindergarten achievement gap by up to a third. The results of the paper are important because “this isn’t something that just arises when boys hit the school system, it’s something that arises pretty early in life,” says Milligan.

“People can point fingers at the school system [the role of curriculum, not enough male teachers] but what we can say is that is clearly those are not the only things going on,” he said. “Rather than focusing all your attention on kindergarten curriculum (from a policy perspective) it is worth thinking about the expectations set in the home and whether society has a role to play in that — say by increasing awareness.”