Bill Cosby has had it up to here with black street culture. "Your dirty laundry gets out of school at 2.30pm every day, it's cursing and calling each other nigger," he recently told a group of black leaders. "They think they're hip. They can't read; they can't write. They're laughing and giggling, and they're going nowhere." The man who is arguably America's most admired black entertainer has turned from the long-suffering dad in "The Cosby Show" into a searing social critic. He dislikes entertainers who play to black stereotypes. He dislikes black street slang ("You can't be a doctor with that kind of crap coming out of your mouth"). And he dislikes parents who blanch at spending $200 on reading programmes but give their children $500 sneakers.

Good on Mr Cosby. There is something of a conspiracy of silence about blacks' dismal performance in school: silence from black leaders who don't want to be accused of "blaming the victim", silence from teachers who don't want to draw attention to the biggest failure of American education. But the achievement gap between blacks and whites is a disgrace.

Black high-school students graduate an average of four years behind white students in basic academic skills. Most black students perform "below basic" in five of the seven key subjects measured by the National Assessment of Educational Progress. These dismal results are putting a ceiling on blacks' upward mobility—a ceiling that is getting ever lower as routine jobs are exported abroad or mechanised out of existence.

The teachers and black politicians blame three standard villains: poverty, prejudice and school funding. A third of black children are brought up in poverty compared with just 13% of white children. Seven in ten black children attend predominantly minority schools, up from 63% in 1980; more than a third attend schools with a minority enrolment of 90-100%. But these villains nevertheless leave a lot unexplained.

Take poverty. American history is full of examples of impoverished immigrants (Jews a century ago, Asians today) who have made it from the inner city to the Ivy League. More worrying still, the achievement gap is just as marked for affluent black children as it is for poorer ones. Or take school funding. There is no simple correlation between education spending and school quality. In "No Excuses: Closing the Racial Gap in Learning", perhaps the best book on this subject, Abigail and Stephen Thernstrom point out that Cambridge, Massachusetts, was left with a huge gap between black and white students despite spending $17,000 for each and every pupil.

Or take prejudice. The percentage of people who tell pollsters that they have nothing in common with people of other races has declined from 25% in 1988 to just 13% in 2003. This is partly because the difference of income between blacks and whites with the same skills in maths and literacy has almost disappeared. Yet, over the same period, the gap in academic achievement has actually widened.

All this suggest that Mr Cosby in on the right track. Researchers routinely explain Asian children's success in terms of Asian cultural values. So why not admit that black children are failing because their culture undervalues success at school—because so many black children dream of becoming sports stars rather than professors, because bookish black children are stigmatised for "acting white", and because almost half of black ten-year-olds spend five hours or more each day watching television?

Mr Cosby's diagnosis of black failure has another great merit. It comes with a remedy attached. Black America once had a flourishing tradition of self-help: the tradition of Frederick Douglass, who was born into slavery but became one of the great orators of his age, and of the army of self-educated blacks who came after him. This tradition was obscured during the civil-rights era as black leaders concentrated on dismantling the machinery of discrimination. But blacks desperately need to revive Douglass's belief in "self-cultivation" if the civil-rights revolution is to amount to something more that a hollow legal shell.

A broadside too narrow
Mr Cosby is well qualified to encourage this revival. He grew up in a poor area of Philadelphia an dropped out of school to join the navy. But he returned to university to take a doctorate in education, and continues tot devote his energies to black improvement, writing books for pre-school readers and pouring money into black colleges.

He has drawn flack, of course. But the real problem with his broadside is that it is too narrow. It is not just black leaders who are failing to hold young blacks to higher standards. It is America in general—and, above all, the educational establishment. Teachers are far too willing to make excuses for black failure, and universities have institutionalised low expectations through affirmative action. Why should black children try as hard as their white peers if they can get into college with lower mark?

There are some signs that America is trying to tackle what George Bush once described as "the soft bigotry of low expectations". The Thernstroms produce plenty of examples of minority schools that are raising academic performance through discipline and accountability. The No Child Left Behind Act—perhaps Mr Bush's most underestimated achievement—is explicitly designed to close the racial achievement gap through a combination of testing and penalties for poor schools.

Changing the attitude of young blacks will not be easy. It is always tempting to idolise celebrities who get paid millions of dollars while misbehaving. And it is always tempting to blame your problems on "society" when "society" has enslaved and disenfranchised your ancestors. But one thing is certain: black America's future will remain dim unless it begins to take Mr Cosby's jeremiads to heart.