TEENAGE girls are hard to impress. Unless you are Michelle Obama, that is. In 2009, on her first foreign trip as First Lady, Mrs Obama visited Elizabeth Garrett Anderson School, a London state (or public) school for girls, about three-quarters of whom are eligible for free school meals. She told a star-struck assembly that she also came from humble origins, explaining how she worked her way from a poor part of Chicago to the Ivy League, a top law firm and (with help from Barack) the White House. “I’m standing here…because of education,” Mrs Obama said. “I thought being smart was cooler than anything in the world.” And unlike many luminaries asked to rouse pupils, the First Lady kept in touch. Mrs Obama invited the girls to see her again in 2011 when she visited Oxford University. There, she told pupils: “All of us believe that you belong here.” One year later a dozen pupils flew over to the White House.

Her message seems to have worked. In a paper published on July 1st, Simon Burgess, an economist at the University of Bristol, analysed the school’s exam results in the years after Mrs Obama’s visits. The 15- or 16-year-olds sitting their GCSEs did much better than girls in the previous year. From 2011 to 2012, for example, the boost was equivalent to each pupil moving from eight C to eight A grades. Those improvements were much bigger than the average increases in performance across London state schools, suggesting that the effects were specific to Elizabeth Garrett Anderson.

It is impossible to prove that Mrs Obama caused the improvement in results. The class of 2012 may have been unusually bright. There may have been a change of teachers. Perhaps it was a fluke. But Mr Burgess thinks not. After controlling for such possibilities and looking at the size of the increase, he concludes: “[The effect] was extremely unlikely to have been generated just by chance.”

Assuming the First Lady did boost results, this could have broader implications for education. The importance of both parents’ and children’s expectations is well established: see the relative success of Asian-American pupils, or Britons of Indian descent. Less understood is how aspirations can be fostered. One approach has been to offer incentives for pupils to work harder and for parents to check on their children. But the work of Roland Fryer, an economist at Harvard University, has found that paying parents and pupils in return for better grades does not always lead to improved results.

A newer set of programmes in American schools, however, tries to foster “intrinsic motivation”, which is more durable. One Chicago scheme, Becoming A Man, involves counselling for troublesome teenage boys in “social-cognitive skills” such as self-control. A recent study found that it boosted school attendance and cut crime. And while the message is important, so too is the messenger. Mrs Obama is not only famous, but also carries legitimacy. As one pupil of Elizabeth Garrett Anderson told the Guardian back in 2011, “She made it. And so can we. That’s not just something we’ve heard now. We really actually felt it.”