BLACK babies born in America are more than twice as likely as white ones to die before their first birthdays. A large racial gap has been present for as long as statistics have been kept. Nonetheless, infant mortality has generally declined at a faster rate for blacks than whites, leading to hope that the disparity might eventually disappear.

That encouraging trend seems to have levelled off. According to a new study published in the Journal of the American Medical Association, black infant mortality in America stopped falling in around 2012. The paper found that if black babies had died at the same rate as white ones, in 2015 nearly 4,000 infant deaths would have been averted.

The main explanation for the racial gap is that black babies are much more likely than white ones to be born prematurely. What leads to prematurity itself is not fully understood: even a tentative cause can be identified in only about half of such births. Risk factors linked to the mother include high blood pressure, smoking, diabetes and obesity. Mothers are also more likely to go into labour too early if they are aged under 20 or over 35.

Black mothers are more likely than white ones to be poor, unmarried and very young—three variables strongly associated with higher infant mortality. These are only partial explanations, though. Studies have found that even after accounting for differences in mothers’ age, education, medical history and use of prenatal care, black women are still more likely than white ones to have babies that are born too early or weighing too little. Babies born to black mothers who have been to university are more likely to die than babies born to white mothers with less than a high-school education. Untangling the medical and social causes would shed light on the reasons—and on what can be done to reduce the mortality rate again.