(Gareth Simpson via Flickr)

Over the past four decades, the number of people locked in U.S. jails on any given day has swelled from about 157,000 to approximately 750,000. They’re mostly men. But the fastest-growing incarcerated group is women.

Since 1970, the number of jailed women has grown 13-fold, from about 8,000 to almost 110,000, according to a new report from the Vera Institute of Justice and the the Safety and Justice Challenge. Nearly 80 percent are mothers of small children, many of them single.

Though the authors said they couldn't determine exactly what's behind the influx of women in prisons, they noted that the financial pressures of single parenthood can lead to minor crimes and that law enforcement has increasingly cracked down on lower-level offenses.

Women who land in a jail cell are more likely than their male counterparts to be the sole guardian of a child, which can make it harder to adhere to a schedule set by a probation officer. For instance, a missed parole meeting could have something to do with a lack of child care.

"Violations may also result from the challenges of juggling supervision requirements with work and family responsibilities," the authors wrote.

This influx of women in jail has coincided with a widespread crackdown on such smaller offenses — simple drug possession, shoplifting, skipping a parole meeting — the kind of offenses officers were more likely to overlook before Richard M. Nixon launched the "war on drugs." A sharp escalation in the enforcement of drug offenses through the '80s and '90s led to greater enforcement of various minor offenses.

"These changes involved an increasingly elastic interpretation of criminal offenses, such as conspiracy — or working together to commit an illegal act — as well as the lowering of thresholds separating minor and serious offenses," according to the report.

Because nonviolent offenses make up a larger portion of the crimes women commit compared with men, women were more heavily affected, the authors argue. Between 1980 and 2009, for example, the arrest rate for drug possession tripled for women and doubled for men.

The rapid increase in women behind bars didn't happen uniformly across the country. Small counties, those with fewer than 250,000 residents, appear to be driving the influx, the report found. The number of women in these jails increased 31-fold from 1970 to 2014.

Since 2000, the jail incarceration rate for women in small counties grew from 79 per 100,000 women to 140 per 100,000. During the same time period, the rate for women in midsize counties increased from 80 to 88. The rate in large counties actually decreased, though, from 76 to 71.

One potential reason: Larger cities generally have more resources to care for poor residents who struggle with mental ailments. Nearly one-third of women in jail live with a serious mental illness (severe depression, bipolar disorder, schizophrenia), compared to roughly a sixth of their male counterparts. A study from the Bureau of Justice Statistics found 75 percent of women in jail said they'd experienced symptoms of a mental health disorder in the past year.

Women in jail also tend to have a history of trauma, data show. Eighty-six percent report having endured sexual violence, the report found. Seventy-seven percent said they'd experienced partner violence.

The authors say policymakers should be aware of the unique challenges women face, such as higher unemployment rates, lower average wages and more mouths to feed on their own. They recommend investing in larger diversion programs, which aim to address root causes of incarceration, rather than promptly placing nonviolent offenders behind bars.

They emphasize that women, men and society, in general, would benefit from a law enforcement shift away from reflexively locking up offenders who don’t pose a threat to others.

“Local jurisdictions should reserve jail incarceration as a last resort for women who are deemed a flight risk or a danger to public safety,” the report states. “Instead, they have allowed jails to become stopgap providers of social services, mental health and substance use assessment and treatment, and temporary housing for women caught up in the justice system — a catchall for those who have slipped through the net of community services, if any exist.”

More on Wonkblog:

What makes the Stanford sex offender's six-month jail sentence so unusual

Poor white kids are less likely to go to prison than rich black kids

There's been a big decline in the black incarceration rate