Simulating Lay Health-Seeking Behavior with LLM Personas and Illness Vignettes: Reproducibility, Prompt Sensitivity, and Slice Dependence

Large language models (LLMs) are increasingly used as “synthetic respondents” to simulate human judgments and decision-making. In healthcare-adjacent settings, a key methodological risk is that simulated behavior may be sensitive to prompt framing, stochastic decoding, and the scenario slice being tested (e.g., red-flag vs non–red-flag situations). We present a fully synthetic, non-human-subject methodological audit in which an LLM, conditioned on a fictional layperson persona, selects a next-action code (A0–A9) for an illness vignette. In a Pilot experiment (40 persona–scenario pairs; two prompt variants; three repeats), action urgency increased with vignette severity and repeatability was moderate (mean modal agreement 0.617). However, within-batch paired prompt comparisons yielded perfect agreement (0/40 mismatches), suggesting that paired designs that do not enforce independence can severely underestimate prompt sensitivity. In an isolated-prompt audit (24 pairs; three repeats), prompt mismatch varied widely across replications (0.0% to 45.8%). To disentangle prompt effects from decoding noise, we performed a controlled follow-up rerun on two slices (non–red-flag and red-flag; 24 pairs each) under explicit decoding settings (temperature 0 vs default temperature 1.0/top-p 0.95) with 8–10 repeats per condition. Prompt sensitivity remained high under both decoding regimes (mean action mismatch 0.787–0.821; mean Jensen–Shannon divergence 0.148–0.196 bits), while near-deterministic decoding improved run-to-run stability in three of four settings. A rubric stress test shifted the action distribution (Jensen–Shannon divergence 0.130) and reduced mean urgency by 1.29 points. Together, these results motivate multi-run, slice-aware, decoding-controlled evaluations when using LLM personas for behavioral simulation.