Visual illusion in male self-assessment of penile dimensions: a clinical study on penile length perception bias between flaccid and erect states.

Background: Despite the significance of penile dimensions in male health and self-perception, there is a lack of population-specific references and understanding of self-assessment biases.

Aim: To establish population-specific references and investigate self-assessment biases in penile dimensions.

Methods: A single-center cross-sectional study (2024-2025) prospectively enrolled 342 Chinese males. Standardized measurements of flaccid and stretched lengths were performed by a trained andrologist. Participants were asked to report their perceived erect penile length, and stratified into three mutually exclusive groups: (1) accurate estimation (AE), where self-reported lengths = stretched lengths; (2) overestimation (OE), where self-reports > stretched lengths; and (3) underestimation (UE), where self-reports < stretched lengths. The penile lengthening ratio (PLR) was calculated as (stretched - flaccid length)/flaccid length. Statistical analyses included paired t tests and one-way analysis of variance for multigroup comparisons.

Outcomes: Mean flaccid and stretched lengths, self-reported lengths, and the distribution of estimation groups were determined.

Results: Mean flaccid and stretched penile lengths were 7.27 ± 1.60 and 11.89 ± 1.57 cm, respectively. Self-reported erectile lengths (12.81 ± 1.85 cm) significantly exceeded measured values (Δ = 0.92 cm, P < .001), with 72.81% of the participants overestimating their erectile length. OE participants exhibited greater flaccid (7.46 ± 1.64 vs 6.74 ± 1.39 cm, P < .05) and stretched lengths (12.01 ± 1.47 vs 11.46 ± 1.69 cm, P < .05) than AE participants. UE individuals showed paradoxically higher stretched lengths (13.50 ± 2.38 vs 11.46 ± 1.69 cm, P < .05) and PLR (97 ± 36% vs 71 ± 14%, P < .05).

Clinical implications: These findings provide critical references for clinical counseling on penile size and perioperative doctor-patient communication, potentially alleviating patient anxiety stemming from cognitive biases to a certain extent.

Strengths and limitations: The strengths include standardized measurements and a clear classification of estimation group. The limitations include incomplete baseline data (lacking penile circumference, smoking history, etc.), single-center small-sample bias, inevitable selection bias, and absence of partner satisfaction data and validated assessments.

Conclusion: This study revealed that self-reported erect lengths among adult males were significantly longer than clinician-measured stretched lengths. OE participants accounted for more than 70% of the sample, while UE participants tended to have a greater PLR. The research provides reference ranges for flaccid and stretched penile lengths in Chinese males, offering objective data to support clinical counseling and surgical communication. This not only alleviates patient anxiety rooted in cognitive biases but also elucidates the potential association between penile size misperceptions and PLR.