To illustrate this, I conducted additional analysis to compare two groups of counties—each with a population of 18.6 million. The first group: Los Angeles County and New York City, which have a combined resident population of 18.6 million in 2014, and are also the largest—and perhaps most notorious—jail jurisdictions in the United States. The second group: 1,003 very small counties, each with between 10,000 and 30,000 residents in 2014, and also with a combined resident population total of 18.6 million (around one-third of all U.S. counties fall into the 10,000 – 30,000 category). Each group holds 6 percent of the total U.S. population, and has grown at nearly the same rate since 1970.

Of course, there are differences between the two groups. The growth of mass incarceration in local jails is one key difference. From the 1970s to the present, NYC and LA’s combined jail population grew 30 percent, from 23,000 to 30,000 people on any given day. This outpaced the cities’ resident population growth of 25 percent. In contrast, in the very small counties, jail populations started out much smaller. For example, Gonzales County, Texas—with 20,000 residents between San Antonio and Houston—had 2 people in jail in 1970. But very small counties grew far more. The jail populations in these very small counties grew six-fold from the 1970s to the present—from 9,000 to 62,000—and now hold double the amount of people behind bars as NYC and LA. Gonzales County had 87 people in jail in 2013, for a jail incarceration rate twice the national average. Or Marion County, Tennessee— with 28,000 residents outside of Chattanooga—had only 8 people in jail in 1970, and now has 131 in 2013.

Another meaningful difference is in diversity: the combined population of New York City and Los Angeles is about 70 percent people of color, and the very small counties are about 80 percent non-Hispanic whites. To understand the full impact of mass incarceration at the local level, it’s important to understand how it affects people of color. Compared to very small counties, far more people of color live in NYC and LA County. One might expect NYC and LA to have more people of color in jail. But they don’t—very small counties have more people of color behind bars on a given day than NYC and LA. While data limits mean we can only compare back to 1990, the changes since then are dramatic. In 1990, 33,000 people of color were behind bars in NYC and LA, but only 9,000 were behind bars in the local jails of very small counties. Twenty-four years later, in 2014, very small counties had tripled to 27,000 and NYC and LA had dropped to 25,000. In some very small counties, the change is dramatic: Custer County, Oklahoma held 11 people of color behind bars in 1990 and 114 in 2013—10 fold growth when the resident people of color population had only doubled.