THAT African Americans are poorer than the American average is well-established. In 2016, the median household income of black Americans was $39,500 compared with $65,000 for non-Hispanic white Americans. Lower parental income and education levels are often cited as reasons for this gap. But a new paper suggests that there is more to it than that. It finds that black men fare worse economically than white men even if they are raised in households with similar incomes and educated similarly. A black boy brought up in a wealthy family is as likely to become poor in adulthood as he is to remain prosperous.

The study, by Raj Chetty at NBER, Maggie Jones at the Census Bureau and two colleagues is notable for its size and sweep: it involves 20m people born between 1978 and 1983–about 94% of all of those born over that period now resident in America. The authors use a range of data on child, parental and neighbourhood characteristics to look at the impact of race on economic mobility over a generation.

It finds that a black man born to parents at the median income would expect to end up lower on the income ladder than his parents, while a white man born to parents of median income would rise by a very small amount. Black women, by contrast, seem to have slightly more upward mobility than white women. In terms of high-school completion, college attendance, incarceration rates, hours worked and employment rates black women also look indistinguishable from white women raised in households with similar incomes. Black men see worse outcomes on all of these measures than both white men and black women. In terms of earnings it appears black women from poor families primarily face barriers because of poverty and gender, while black men from those families face barriers because of poverty and race.

The paper’s authors dismiss some frequently cited explanations for the mobility gap. They find that the impact of growing up in a single-parent home or one with less-educated parents on upward mobility is muted. And while neighbourhoods matter to incomes, they have little impact on the mobility gap, which is present within and across nearly all of the 28,850 neighbourhoods in the study. Among children with parents earning at the 25th percentile, black boys had lower incomes in adulthood than white boys in 99% of the neighbourhoods. School and housing integration may not help if the mobility gap persists even among children raised on the same block, going to the same school.

The researchers note that in low-poverty neighbourhoods two things do improve black boys’ prospects and close the black-white gap: “low levels of racial bias among whites and high rates of father presence among blacks”. The presence of fathers seems to make a difference to a boy regardless of whether his own father is present, suggesting that other fathers can act as role models and mentors. The researchers point out that few black children grow up in such places. Fewer than 5% of black children grow up in areas with a poverty rate lower than 10% and more than half of black fathers present. By comparison, 63% of white children grow up in areas with such conditions.

In all this, it is hard to ignore the role the criminal-justice system plays in the lives of black males. Black drivers are three times more likely than white drivers to have their car searched by police after a stop, blacks are arrested twice as often as whites for drug crimes despite similar usage rates, they are more often offered plea deals which involve jail time for similar crimes, and black males in particular are given longer sentences for the same crime.

Once they leave jail, black men face greater barriers with re-integration. Devah Pager from Princeton University and colleagues sent teams of black and white men to apply for jobs in New York, presenting equivalent resumés differing only in race and criminal background. They found a significant negative effect of a criminal record on employment outcomes that was substantially larger for African Americans.

This evidence of discrimination in criminal justice can go some way towards explaining the results of the latest research. The overall incarceration rate for black men in its sample is 10.3% compared with 1.6% for white men and 0.6% for black women. That suggests an inter-generational impact of high rates of father incarceration on outcomes for boys. Other studies have found that schools with high levels of incarceration among parents produce worse educational outcomes for children.

To the extent that incarceration is one factor behind the income gap between white and black American men, there is some good news: prison populations have been falling. There has been a 17% decline in the number of black inmates in state and federal prisons between 2009 and 2016, outpacing the overall decline in incarceration rates.