Over the past four decades, there has been a nearly five-fold increase in the number of people in U.S. jails: the approximately 3,000 county or municipality-run detention facilities that primarily hold people arrested but not yet convicted of a crime.

From just 157,000 people held on any given day in 1970, the jail population rose to 745,000 people by 2014.

Intended to house those deemed to be a danger to society or a flight risk before trial, jails have become massive warehouses primarily for those too poor to post even low amounts of bail or too sick for existing community resources to manage. With more than 11 million admissions annually, the role jails play as a principal driver of mass incarceration is receiving increasing attention from policymakers and the public.

Despite this scrutiny, one aspect of this growth has received little attention: the precipitous rise in the number of women in jail. Although they generally fare better than men in pretrial decisions, the number of women incarcerated in jails is growing at a faster rate than any other correctional population.

Since 1970, the number of women in jail nationwide has increased 14-fold—from under 8,000 to nearly 110,000— and now accounts for approximately half of all women behind bars in the United States.

Once a rarity, women are now held in jails in nearly every county—a stark contrast to 1970, when almost three-quarters of counties held not a single woman in jail.

Surprisingly, small counties (those with 250,000 people or fewer in 2014) have been the main engine of this growth, with the number of women in small county jails increasing 31-fold from 1970 to 2014.

Since 2000, jail incarceration rates for women in small counties have increased from 79 per 100,000 women to 140 per 100,000 women. In contrast, mid-sized counties’ jail incarceration rates for women only grew from 80 to 88 per 100,000 women, while rates in large counties actually decreased from 76 to 71 per 100,000 women within that same timeframe. Today, nearly half of all jailed In 1970, 73 percent of counties across the U.S. reported zero women in their jails.