What's Driving Increases in the Cook County Jail Population?

Cook County Jail Population

Immediately after the implementation of the Pretrial Fairness Act (PFA) the population of the Cook County jail decreased. Between September 17, 2023 (the day before the implementation of the PFA) and January 1, 2024, the jail population decreased 14% - or nearly 800 people. Much of this decrease was the result of fewer admissions to jail - between September 2023 and December 2023, monthly jail admissions fell 15% - or roughly 280 fewer people admitted per month.

However, the lower jail population and admissions was short-lived. Between January 2024 and June 2025, the population of the Cook County jail increased 28% - adding just over 1,200 people to the jail population. By the end of June 2025, the jail population exceeded 5,800 people for the first since 2022. And the jail population continues to rise - surpassing 6,000 people by the end of September 2025.

During this same time, admissions to the jail increased. Between the last quarter of 2023 (immediately after the implementation of the PFA) and the second quarter of 2025, the number of people admitted quarterly to the Cook County jail increased 40% - or nearly 1,900 people. By June 2025, over 6,500 people were admitted to the jail in a three-month period. In the first three months of 2025, over 6,800 people were admitted to jail - the highest quarterly admissions since the first quarter of 2020 just before COVID-19.

What is driving this recent increase in the Cook County jail population? This report considers changes in admissions to the Cook County jail as one driver of the jail population.

Jail admissions can be influenced by trends in the number of criminal cases filed, the rates at which pretrial detention is sought by the state's attorney through detention petitions, and the rates at which these petitions are "successful" - in other words, the rate at which they are granted by a judge and result in detention. Each of these factors - cases filed, petitions to detain, and successful petitions to detain - have increased over time and contribute to the recent increase in the Cook County jail population.

To estimate the impact of these factors, we examine cases that could result in pretrial detention at initial appearance. Under the PFA, a limited set of offenses are detainable at initial appearance under either the public safety consideration or the willful flight consideration. Analyses indicate that individuals rarely are detained under the willful flight consideration. As such, we consider only cases detainable under the public safety consideration, which includes "forcible" felonies, “nonprobationable” felonies that would require imprisonment upon conviction, and misdemeanor and felony domestic battery.

Cases Filed

Following the implementation of the PFA, there was an increase in the number of detainable cases filed in Cook County; however, this closely followed regular seasonal patterns in filing trends. Looking at year-over-year case filings shows relative stability. For example, in the first half of 2024, roughly 10,440 detainable cases were filed in Cook County; in the first half of 2025, just over 10,300 detainable cases were filed.

To illustrate the potential impact of changes in case filings, we compared the actual number of case filings to the projected number of case filings based on the prior year's trend. The difference between the actual number of case filings and the projected number provides a rough estimate of the potential impact of changes in case filings on admissions. The shaded area in the chart below shows the difference between these two numbers. The number of detainable cases filed was higher than expected at the end of 2024, but was lower than expected at the beginning of 2025.

Petitions to Detain

The PFA requires that individuals - including those charged with a detainable offense - be released on their own recognizance (with or without supervision), unless the state's attorney files a petition to detain. Since the implementation of the PFA, the rate at which a petition to detain is filed has increased. In the first year after the PFA, the state's attorney filed a petition to detain in roughly 30% of detainable cases. However, at the end of 2024, the rate started to increase; by June 2025, the state's attorney filed a petition to detain in roughly 44% of detainable cases. Thus, even if there were no change in the number of cases filed or the rate at which detention is imposed, the increase in the rate at which petitions to detain are filed would partially contribute to the increase in jail admissions and jail populations.

To illustrate the potential impact of changes in the rate at which petitions to detain are filed, we compared the actual number of cases with a detention petition filed to the projected number of cases with a detention petition filed. We calculated the number of projected cases for the last quarter of 2024 and the first two quarters of 2025 - those in which the rate was higher than in the prior year; we used the average of petition rates from October 2023 through September 2024 (when rates were relatively stable) for the projected calculation. The difference between the actual number of cases with a detention petition filed and the projected number provides a rough estimate of the potential impact of changes in detention petition rates on admissions. The shaded area in the chart below shows the difference between these two numbers.

The number of detainable cases with a detention petition filed was significantly higher than expected through June 2025. For example, in the second quarter of 2025, the higher rate of petitions to detain led to 847 more detention petitions filed than expected.

"Successful" Petitions to Detain

Once the state's attorney files a petition to detain, a judge must determine that detention is necessary and grant the petition, resulting in a defendant being detained. Since the implementation of the PFA, the rate at which petitions to detain are "successful" has increased. In the first year after the PFA, judges granted petitions to detain and imposed detention in roughly 60% of cases in which a petition to detain was filed. However, in 2025 the rate started to increase; by June 2025, judges granted petitions in roughly 67% of cases in which a petition was filed. Thus, even if there were no change in the number of cases filed or the rate at which petitions to detain are filed, the increase in the rate at which these petitions are "successful" would partially contribute to the increase in jail admissions and jail populations.

As above, to illustrate the potential impact of changes in the rate at which petitions to detain are granted, we compared the actual number of cases with a successful detention petition to the projected number of cases with a successful detention petition granted. We calculated the number of projected cases for the last quarter of 2024 and the first two quarters of 2025 - those in which the rate was higher than in the prior year; we used the average of successful petition rates from October 2023 through September 2024 (when rates were relatively stable) for the projected calculation. The difference between the actual number of cases with a successful detention petition and the projected number provides a rough estimate of the potential impact of changes in successful detention petition rates on admissions. The shaded area in the chart below shows the difference between these two numbers.

The number of detainable cases with a successful detention petition was slightly lower than expected in the last quarter of 2024 - roughly 84 fewer than expected. However, it was higher than expected in 2025; for example, in the second quarter of 2025, the higher rate of successful petitions to detain led to 158 more individuals detained than expected.

Estimating the Impact of Changes in Filings, Petitions, and Success Rates

To estimate the potential impact of changes in case filings, petitions to detain, and successful detention petitions, we combine the differences between the actual and estimated numbers above. In the chart below, the blue line indicates the actual number of jail admissions and the red dashed lines indicate the estimated number of jail admissions without changes in cases filed, petitions to detain, and successful detention petitions. Overall, changes in the number of cases filed had no impact on the number of jail admissions - indicated by the very similar trends in the lines. Changes in the rate of successful petitions to detain had a small impact on the number of jail admissions. However, changes in the rate at which petitions to detain were filed markedly increased jail admissions - indicated by the difference between the lines.

Between the first half of 2024 and the first half of 2025, jail admissions increased by 3,084 individuals. Of this increase, changes in petitions to detain account for 1,068 of these individuals - or 34.6% of the increase; changes in successful petitions to detain account for another 201 of these individuals - or 6.5% of the increase. Thus, changes in petitions to detain and successful petitions to detain account for roughly 41% of the increase in jail admissions in the first half of 2025. The rest of the increase in jail admissions is due to other factors - factors related to cases that are non-detainable under the public safety consideration.

Other Factors Driving the Jail Population

Between the first half of 2024 and the first half of 2025, jail admissions increased by 3,084 individuals. While changes in the processing of cases detainable under the public safety standard account for 1,269 of these additional admissions (41%), the remaining 1,815 admissions are related to cases that are non-detainable under the public safety consideration.

A very small proportion is accounted for by increases in cases filed, petitions to detain, and successful detention petitions for cases detainable under the willful flight consideration; overall, these changes account for 96 admissions (3% of the increase). An additional 1,143 admissions (37% of the increase) are due to individuals admitted shortly after their first appearance but not charged with a detainable offense, which may indicate individuals revoked from pretrial release. The remaining 576 admissions (19% of admissions) are likely due to revocations of parole or probation or warrants from other jurisdictions.

Conclusion

While jail populations are determined by admissions and length of stay, it appears that almost all the increase in the Cook County jail population over the past 18 months has been due to increased admissions. It does not appear that any of the increase in admissions can be attributed to changes in the number of cases filed that are eligible for pre-trial detention under the public safety consideration. Rather, a large portion (41%) of the increased admissions, and thus the jail population, was the result of changes in practice - specifically, an increased rate of detention petitions filed by the Cook County State’s Attorney’s Office and an increased rate of detention petitions granted by judges for those charged with detainable offenses under the public safety consideration. Despite these increased rates of detention petitions filed and granted, the current rates in Cook County are generally consistent with those seen across the rest of Illinois.

The next significant portion (37%) of the increased jail admissions (and thus the population) consists of individuals admitted to the jail shortly after their first appearance, but who were not charged with a detainable offense under the public safety consideration. It is likely that most of these individuals were admitted to jail because of either their failure to attend court hearings or new charges being filed while they were on pretrial release. Under the PFA, individuals who are not charged with a detainable offense can still be admitted to jail, following a hearing, for up to 30 days as a sanction if they fail to attend required hearings. Similarly, if an individual released pretrial is charged with a new Class A misdemeanor or felony offense, even if that new offense is not detainable under the public safety consideration, they can have their pretrial release revoked following a hearing and be admitted to jail until either their case is disposed, or the judge reconsiders their detention status. For example, retail theft, which is not detainable under the public safety consideration, experienced a large increase in jail admissions; jail admissions for retail theft increased from 709 admissions during first half of 2024 to 1,437 admissions in first half of 2025. This increase of more than 700 admissions accounted for 25% of the overall increase in admissions to the jail during that time and likely reflects cases that were sanctioned for failing to appear or defendants on pretrial release who were charged with new Class A misdemeanor of felony charges.

That said, it is important to note that the vast majority of those in the Cook County jail—both before and after the PFA—were charged with either violent crimes or weapon offense (e.g., illegal possession of a firearm). During the 2nd quarter of 2023 (prior to the PFA), 64% of those in the jail on any given day were charged with a violent crime and an additional 18% with a weapon offense, or 82% combined; during the 2nd quarter of 2025, when the jail population experienced a significant increase, 64% of those in the jail on any given day were charged with a violent crime and an additional 20% with a weapon offense, or 84% combined.

Support

This research is supported by Award No. 15PNIJ-21-GG-02807-RESS awarded by the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice. The opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect those of the Department of Justice. This research is also generously supported by The MacArthur Foundation.