In the USA, people with mental health conditions are overrepresented in prisons. A review of several studies found that more than half of prison inmates had at least one mental health condition, while only 11% of the general population has a mental health condition (Hoke, 2015) and other studies have reported similar rates of mental illness among prison inmates (Al-Rousan et al., 2017; Appiah-Kusi et al., 2022). In terms of specific conditions, different studies have more variability in terms of prevalence.

Rates of Specific Conditions

Attention Deficit Hyperactivity Disorder: A systematic review of studies found that 10-25% of inmates were diagnosed with ADHD and that ADHD was the most common mental disorder in inmates (Prins, 2014). Another study found that 66.67% (2/3) of inmates had a childhood history of ADHD (Appiah-Kusi et al., 2022). Hoke (2015) also noted a higher-than-normal presence of ADHD in inmates, but did not provide an exact percentage.
Autism Spectrum Disorder: Appiah-Kusi et al. (2022) estimate that up to 10% of prison inmates may have an autism spectrum disorder. Hoke (2015) also mentioned higher-than-average rates of autism spectrum disorders in prison inmates.
Depression: 10-18% of inmates were diagnosed with depression in two studies (Al-Rousan et al., 2017; Hoke, 2015) and another study found that 50% of inmates had a lifetime history of depression (Appiah-Kusi et al., 2022).
Personality disorders: Hoke (2015)'s review of multiple studies found that 65% of male inmates had a personality disorder, along with 42% of female inmates. This same review found that 47% of male prison inmates and 21% of female inmates specifically had Antisocial Personality Disorder. Prins (2014) noted a significantly higher rate of Borderline Personality Disorder than in the general population.
Post-Traumatic Stress Disorder: A review of studies done by Prins (2014) found that 15-48.2% of female inmates had PTSD. Appiah-Kusi et al. (2022) also noticed a higher rate of PTSD in those attending court than in the general population.
Psychosis:A review of several studies by Hoke (2015) found that 3-7% of male inmates and 4% of female inmates had a psychotic disorder, while Appiah-Kusi et al. (2022) found that 33.33% (1/3) of inmates had a lifetime history of psychosis.
Substance Use Disorder: A study by Al-Rousan et al. (2017) found that 26% of inmates had a history of substance use disorder. Hoke (2015) found that 56-70% of inmates would meet the criteria for a substance use disorder. Both sources also found higher rates of substance abuse in inmates with other mental health conditions than in those without.