The pattern doesn’t hold for every city. Black areas in Houston, Dallas, Philadelphia and Washington seem to support the local team at the same rate as other areas. But it also exists in some places where it is not evident at first glance, like in Detroit, where support for the Pistons often drops by 10 percentage points on the south side of 8 Mile Road.

None of this means that hometown teams lack black fans. Given how many N.B.A. fans are African-American, teams like the Pistons, the Hawks and the Bulls clearly have large numbers of black fans. In fact, a larger percentage of black Chicagoans than white Chicagoans probably consider themselves Bulls fans. But the pattern does suggest that an even larger share of Lakers and Heat fans may be African-American.