Social anxiety disorder (SAD) patients may have self-referential ideas and share other cognitive processes with paranoid delusional disorder (PDD) patients. From an evolutionary perspective, SAD may derive from biologically instinctive social hierarchy ranking, thus causing an assumption of inferior social rank, and thus prompting concerns about mistreatment from those of perceived higher rank. This naturalistic longitudinal study followed four patients with initial SAD and later onset of PDD. These four patients show the same sequence of diagnosed SAD followed by diagnosed PDD, as is often retrospectively described by other PDD patients. Although antipsychotic medication improved psychotic symptoms in all patients, those who also had adjunctive serotonin-specific reuptake inhibitors for SAD had much more improvement in both psychosis and social functioning. From an evolutionary perspective, it can be conjectured that when conscious modulation of the SAD social rank instinct is diminished due to hypofrontality (common to many psychotic disorders), then unmodulated SAD can lead to paranoid delusional disorder, with prominent ideas of reference. Non-psychotic SAD may be prodromal or causal for PDD.