This Economist article argues it’s a matter of body chemistry.

The weaker sex

“IT’S all to do with their brains and bodies and chemicals,” says Sir Anthony Seldon, the master of Wellington College, a posh English boarding school. “There’s a mentality that it’s not cool for them to perform, that it’s not cool to be smart,” suggests Ivan Yip, principal of the Bronx Leadership Academy in New York. One school charges £25,000 ($38,000) a year and has a scuba-diving club; the other serves subsidised lunches to most of its pupils, a quarter of whom have special needs. Yet both are grappling with the same problem: teenage boys are being left behind by girls.

It is a problem that would have been unimaginable a few decades ago. Until the 1960s boys spent longer and went further in school than girls, and were more likely to graduate from university. Now, across the rich world and in a growing number of poor countries, the balance has tilted the other way. Policymakers who once fretted about girls’ lack of confidence in science now spend their time dangling copies of “Harry Potter” before surly boys. Sweden has commissioned research into its “boy crisis”. Australia has devised a reading programme called “Boys, Blokes, Books & Bytes”. In just a couple of generations, one gender gap has closed, only for another to open up.

The reversal is laid out in a report published on March 5th by the OECD, a Paris-based rich-country think-tank. Boys’ dominance just about endures in maths: at age 15 they are, on average, the equivalent of three months’ schooling ahead of girls. In science the results are fairly even. But in reading, where girls have been ahead for some time, a gulf has appeared. In all 64 countries and economies in the study, girls outperform boys. The average gap is equivalent to an extra year of schooling.