Why do white working class children underachieve in schools?
MPs on the education select committee delivered the verdict that they are "consistently the lowest performing group in the country".
It matters a lot - as Ofsted boss Sir Michael Wilshaw says - because this is the biggest single group of underperformers.
If there is any chance of England's school results catching up with international competitors, these children will need to get better results.
"If we don't crack the problem of low achievement by poor white British boys and girls, then we won't solve the problem overall," said Sir Michael.
What makes this a more complex question is that it isn't explained by the catch-all of poverty. Children from poor Indian, Pakistani, African and Caribbean families do much better, despite similar levels of disadvantage.
And this reinforces the message of the international Pisa tests, that there is nothing inevitable about the link between coming from a poor background and low achievement in school.
The reason that Shanghai comes top is that there is an assumption that all children will reach a decent standard of education, and that excellence isn't a rationed resource.
So what is going on with this group of poor white youngsters?
It might mean getting into territory that is much less easy to measure, under the heading of "cultural differences".
Leicester City Council's evidence to the committee talked of a white working class culture with "low aspirations and negative attitudes towards education".
What's really depressing is the idea that working class culture should be seen as something negative and demoralised.
It wasn't that long ago that the white working class was seen as vibrant and ambitious. Fashion, rock and roll bands and large slices of mainstream culture were shaped by their tastes.
When John Lennon told people in the posh seats to "rattle your jewellery", he didn't mean it as a compliment. The long lineage of English pop bands such the Kinks, the Jam, the Clash and the Smiths drew upon the energy of working class culture.
These bands were also about a different kind of education - a self-taught, eclectic form of learning. You might begin with a Dexys Midnight Runners album and end up reading Laurence Sterne. The Smiths brought Oscar Wilde to bored provincial teenagers, the Jam quoted Shelley on album covers.
And when we talk about culture, how many cultural organisations are no longer part of white working class life? Institutions which provided a whole eco-system of ambitions, ideas and wider horizons - trade unions, churches, social clubs, sports clubs, public libraries - have been marginalised.
The pubs have been shut and local high streets are filled with pay-day lenders, pawnshops and poundshops.
The biggest change of all, highlighted by the select committee report, is the changing jobs market.
The "bedrock" of traditional industrial and manufacturing jobs has been broken up, MPs were told. Many big employers were communities in their own right - and again provided links to social clubs and informal learning.
If the modern version of working class means being caught in low-wage, fragile employment, predated by debt and insecurity, what does that mean for children growing up in those communities? Who do they look up to? What does this do to people's health and the well-being of family life?
Outflanked by the financial muscle of the middle classes and by education-hungry, ambitious immigrants, it doesn't leave much left.
MPs have identified particular geographical patterns for white working class underachievement - such as coastal towns.
But there is also a cultural isolation. A study last year showed that a third of young unemployed youngsters rarely left their own homes, marooned by the shifting economic sands.
There have also been major demographic changes. In London, where the white working class pupils have the least bad results, they are now a small minority.
In the 14 central London boroughs, fewer than one in five primary school children are in the category of "white British". The working class will be an even smaller minority within that minority. The numbers are working against their influence.
Another set of figures published on Wednesday showed the pivotal link between education, aspiration and jobs.
The Office for National Statistics showed that people without any qualifications were twice as likely to be unemployed as those with a qualification. That one single piece of paper, passing a single exam, halved the likelihood of unemployment.
Education has become the gateway to jobs, more than ever before. And being locked out of education will have a higher cost than ever before.