  This study examines the relationship between homelessness and recidivism
among people on probation with and without behavioral health problems. The
study also illustrates a new way to summarize the effect of an exposure on an
outcome, the Incremental Propensity Score (IPS) effect, which avoids pitfalls
of other approaches commonly used in criminology. We assessed the impact of
homelessness at probation start on rearrest within one year among a cohort of
people on probation (n = 2,453). We estimated IPS effects, considering general
and crime-specific recidivism if subjects were more or less likely to be
unhoused, and assessed effect variation by behavioral health problem status. We
used a doubly robust machine learning estimator to flexibly but efficiently
estimate effects. A substantial intervention -- reducing homelessness by
roughly 65% -- corresponded to a 9% reduction in the estimated average rate of
recidivism (p < .05). Milder interventions showed smaller, non-significant
effect sizes. Stratifying by behavioral health problem and rearrest type led to
similar results without statistical significance. Minding limitations related
to observational data and generalizability, this study suggests large
reductions in homelessness lead to significant reductions in rearrest rates.
Efforts to reduce recidivism should include interventions that make
homelessness less likely, but notable differences in recidivism will require
these interventions be sizable. Meanwhile, efforts to establish recidivism risk
factors should consider alternative effects, like IPS effects, to maximize
validity and reduce bias.
