An Agent-Based Explanation for Mentally Ill Patient's Living Situation Changes

An Agent-Based Explanation for Mentally Ill Patient's Living Situation Changes


A Severely and Persistently Mentally Ill (SPMI) patient is generally defined as someone with a diagnosis of Schizophrenia, Bipolar Disorder, or Major Depressive Disorder, and this group constitutes about 1.7% of the US population. The housing environment of the SPMI population in the United States has changed drastically over the past 60 years, most notably in the percentage of the population living in prison versus the percentage living in community based care and private residences.

Many of the changes have been successful. The number of SPMI patients living in the community increased, and although SPMI patients can still be institutionalized against their will, the likelihood of this happening is much lower than in the past. Yet a growing minority of people with severe illness are worse off because they are homeless or incarcerated.

In this case study, IBM Global Research and Otsuka Pharmaceuticals used an agent-based approach to model these remarkable swings in SPMI living situations over the second half of the 20th century. They wanted to better understand the situation and possible improvements that could be made.


The important thing to note about SPMI housing options, is that some types of housing situations (jails, prisons and long-term hospitals) keep SPMI patients longer when they have mental health relapses, while the majority of housing situations seek to evict patients who misbehave in this same way. This fundamental difference was the key rule that the model sought to demonstrate as a major influencing force on the dramatic housing situation changes undergone by SPMIs over the second half of the 20th century.

SPMI Housing Situation Changes

SPMI Housing Situation Changes

The SPMI housing agent-based model contained components based upon the SPMI population’s real world housing environment.

First, the model included the “housing cycle”. When SPMIs left one housing situation, they were faced with a decision about where to live next. SPMI housing choices were thus a continuous cycle. Upon a patient’s arrival at the housing decision point, the patient was randomly placed into one of the seven housing types with certain probabilities. The housing types included:

The housing types had baseline lengths of stay for SPMI patients. Long term hospitals tended to keep SPMI patients for longer than community hospital emergency rooms, etc.

The SPMI patient agents cycled between the decision point and actually residing in the location decided upon. Each patient had a “time to mental health crisis”, representing the various severity levels of mental illness. Time to crisis was uniformly distributed between 5 and 250 days for each SPMI agent in the model and was static for each patient for the entire model run. In this model, a mental health crisis was defined as a symptom flare-up that would make it clear to people interacting with the agent that the agent was not mentally stable. Examples included a psychotic episode, hallucinations, deep depression, etc. In the model animation, agents were closer to the color pure red when their time to crisis was closer to the minimum, and closer to the color pure green when it was closer to the maximum. The model was populated with 1,000 such SPMI patient agents.

The seven housing types responded to a SPMI patient’s mental health crisis differently. While a long-term hospital or a prison typically extended length of stay for a patient having a crisis, most other types of housing sought to evict or discharge such patients.

Healthcare Agent Based Simulation Model Structure

The housing cycle of SPMI agents

The model also included a ceiling placed by the United States legislation on the percentage of patients who can possibly live in long-term hospitals. The ceiling was gradually lowered each decade, which reflected what happened in real life. With each subsequent lower ceiling on the percentage of agents capable of being housed in long term hospitals, private housing added the majority of the formerly hospitalized agents with a small increase to the jail and prison populations.

The model metrics included the baseline lengths of stay and likelihood of going to each housing type upon reaching the housing decision point. Included were the date, percentage of the total population currently living in each housing situation, and the average time to crisis for the group currently living in each of the seven types of housing.

The model closely matched real world experience about the health of SPMI people living in each of the housing categories. The least healthy SPMI patients tended to pool in long-term hospitals and prisons. As long-term hospitals lost capacity, the worst SPMI agents increasingly moved into jails and prisons.

The reason for this phenomenon of the lowest time to crisis agents pooling in long-term hospitals and prisons had to do with the unique way in which these two housing types treated patients who have a crisis while housed in their custody. The less healthy the person, the stronger the reinforcing effect to stay in a long term hospital or prison and to leave all the other housing options. An ethical problem arises when legislation places a limit on the number of SPMI patients who can live in long term hospital, resulting in prison being the only housing environment which has both the capacity and a response of increased length of stay upon a mental health crisis event.


This agent-based model improved the understanding of Severely and Persistently Mentally Ill housing dynamics in multiple important ways.

First, the model showed that even a weak length of stay alteration due to SPMI crises produced a very strong effect. SPMI patients were in a state of flux until arriving in a place that will not let them leave.

Second, increasing patient time to crisis has a significant positive impact on the population’s housing makeup. In a second model run, the researchers increased patient time to crisis by 45 days in the year 2000 and examined what would happen by 2030. As expected, a healthier population (a population with higher time to crisis values) had weaker length of stay reinforcing effects and thus lowered the prison population rate. This insight had implications for policy makers looking to evaluate the return on investment of projects designed to improve the lives of the mentally ill. This agent-based simulation allowed them to understand that proactive governments can indeed lower prison rates by improving mental health in a geographic area.

Finally, the model raised ethical concerns for future mental health policies in the United States. The suggested ways to improve the situation were either increase long term hospital capacity or find a way to increase patient time to crisis. One effort to improve SPMI patient health in the information technology realm is currently underway with a partnership between Otsuka Pharmaceuticals and IBM. This project seeks to use care coordination information technology to help a geographic area’s many health care providers work together to efficiently treat SPMI patients.

More details on the project can be found in the accompanying article or the video presentation by Kyle Johnson from IBM Global Business Services:

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