The purpose of this study is to examine how Bayesian belief updating changes throughout psychotherapeutic treatment for persecutory delusions. Specifically, individuals with a psychotic disorder diagnosis who endorse both a current persecutory delusion with strong conviction and significant worry will be recruited and randomized to receive either a CBT-based worry intervention for persecutory delusions or an active control condition (befriending therapy). The investigators will examine: 1) whether belief updating parameters change as delusion severity changes, 2) whether CBT contributes to greater change in belief updating parameters than befriending therapy, and 3) whether neural correlates of belief updating parameters, as measured using functional magnetic resonance imaging (fMRI), predict treatment response.

Inclusion Criteria:

1. Men and women age 18 - 65.
2. Communicative in English.
3. Premorbid Intelligence >79 (WTAR)
4. Provide voluntary, written informed consent.
5. Physically healthy by medical history.
6. Weight <300 lbs
7. Stable medication regimen over at least the past two weeks, including the use of either an oral or intramuscular administration of an antipsychotic medication.
8. Diagnosis of a non-affective psychotic disorder (schizophrenia, schizoaffective disorder, schizophreniform disorder, delusional disorder, brief psychotic disorder, psychosis NOS) confirmed by Structured Clinical Interview for Diagnostic and Statistical Manual (DSM)-5 (SCID) or diagnostic interview with a trained clinician.
9. A persecutory delusion scoring at least a 3 on the conviction scale of the Psychotic Symptoms Rating Scale (PSYRATS) that had persisted for at least two weeks and that was not considered the direct result of substance use
10. A clinically significant level of worry, as shown by a score of at least 44 on the Penn State Worry Questionnaire (PSWQ).

Exclusion Criteria:

1. Age less than 18 or greater than 65.
2. Not communicative in English.
3. Premorbid IQ < 79 (WTAR)
4. Unable to provide written informed consent.
5. Current medical or neurological illness.
6. History of severe head trauma.
7. Weight >300 lbs
8. Primary diagnosis of alcohol or substance use disorder or personality disorder
9. Conditions that preclude fMRI scanning (as defined in the fMRI Screening Form)
10. Subjects who are actively involved with individual cognitive therapy (Past experience with individual therapy is not an exclusion)