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    World J Mens Health. 2023 Oct;41(4):848-860. English.
    Published online Feb 15, 2023.
    https://doi.org/10.5534/wjmh.220203
    Copyright © 2023 Korean Society for Sexual Medicine and Andrology
    Original Article

    Worldwide Temporal Trends in Penile Length: A Systematic Review and Meta-Analysis

    Federico Belladelli,1,2,3 Francesco Del Giudice,4,5 Frank Glover,5 Evan Mulloy,3 Wade Muncey,3 Satvir Basran,3 Giuseppe Fallara,1,2 Edoardo Pozzi,1,2 Francesco Montorsi,1,2 Andrea Salonia,1,2 and Michael L. Eisenberg3
      • 1Division of Experimental Oncology/Unit of Urology, URI, IRCCS Ospedale San Raffaele, Milan, Italy.
      • 2Department of Urology, University Vita-Salute San Raffaele, Milan, Italy.
      • 3Department of Urology, School of Medicine, Stanford University, Stanford, CA, USA.
      • 4Department of Urology, University Sapienza, Rome, Italy.
      • 5Gangarosa Department of Environmental Health, Rollins School of Public Health, Emory University, Atlanta, GA, USA.
    • Correspondence to: Michael L. Eisenberg. Department of Urology, School of Medicine, Stanford University, 450 Serra Mall, Stanford, CA 94305, USA. Tel: +1-650-725-5746, Fax: +1-650-498-5346, Email: eisenberg@stanford.edu
    Received September 26, 2022; Revised December 15, 2022; Accepted December 20, 2022.

    This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    Abstract

    Purpose

    Normative male genital measurements are clinically useful and temporal changes would have important implications. The aim of the present study is to characterize the trend of worldwide penile length over time.

    Materials and Methods

    A systematic review and meta-analysis using papers from PubMed, Embase, and Cochrane Library from inception to April 2022 was performed. PRISMA guidelines were used for abstracting data and assessing data quality and validity. Pooled means and standard deviations for flaccid, stretched, and erect length were obtained. Subgroup analyses were performed by looking at differences in the region of origin, population type, and the decade of publication. Metaregression analyses were to adjusted for potential confounders.

    Results

    Seventy-five studies published between 1942 and 2021 were evaluated including data from 55,761 men. The pooled mean length estimates were flaccid length: 8.70 cm (95% CI, 8.16–9.23), stretched length: 12.93 cm (95% CI, 12.48–13.39), and erect length: 13.93 cm (95% CI, 13.20–14.65). All measurements showed variation by geographic region. Erect length increased significantly over time (QM=4.49, df=2, p=0.04) in several regions of the world and across all age groups, while no trends were identified in other penile size measurements. After adjusting for geographic region, subject age, and subject population; erect penile length increased 24% over the past 29 years.

    Conclusions

    The average erect penis length has increased over the past three decades across the world. Given the significant implications, attention to potential causes should be investigated.

    Keywords
    Anatomy; Hormones; Meta-analysis; Penis

    INTRODUCTION

    As male sexual dysfunction diagnoses and treatments are common [1, 2], penile size remains important [3]. Penile size has been suggested to associate with sexual strength, virility, and vitality in men [4], as well as a man’s self-esteem [5].

    The penis is formed during gestation under hormonal influences and continues to grow through puberty [6]. Investigators have reported changes in normal male genital development over time as assessed by falling sperm counts, declines in serum testosterone levels, higher rates of testicular tumors, and increasing genital birth defects [7, 8, 9, 10]. While the etiology of reported changes is uncertain, many have hypothesized environmental changes as potential culprits [7, 11].

    Penile size has been measured in several studies but no comprehensive study exists to examine geographic variation or temporal trend [12, 13, 14]. The aim of this systematic review and meta-analysis is to critically evaluate the literature to report the trend of penile length over time and in different geographic regions.

    MATERIALS AND METHODS

    1. Evidence acquisition

    The protocol for this systematic review and meta-analysis was registered in PROSPERO (registration number: CRD42022335620). This meta-analysis was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The following research question was established based on the PICO criteria [15]: Has penile length changed over time globally? We performed a systematic review of the literature in PubMed, Embase, and Cochrane from inception to April 2022, to identify studies that evaluated penile size. Search terms included: “Penile Length” OR (“Width” OR “Circumference” OR “Dimension”) AND (“Erect” OR “Flaccid” OR “Stretched”). The reference lists of the included studies were also screened for relevant articles. Seventy-five original articles were included and critically evaluated.

    2. Selection of the studies and criteria for inclusion

    This analysis was restricted to data collected from original articles that examined men’s penile length. Studies were considered eligible if the quantitative measurement of penis size was measured by an investigator, the sample included ≥10 participants, participants were aged ≥17 years, and if they provided sample size, mean, and standard deviation (SD) of flaccid or erect length measured from the root (pubo-penile junction) of the penis to the tip of the glans (meatus) on the dorsal surface. Articles were excluded if they were based on a self-measurement and if they reported measurements done after major pelvic surgery. Abstracts and meeting reports were excluded from the analysis.

    Two authors (FB and ME) independently screened the titles and abstracts of all articles. Abstracts and full-text articles were examined independently by five authors (FB, FDG, EM, ME, and FG) to determine whether or not they met the inclusion criteria. Final inclusion was determined by the consensus of all investigators. Selected articles meeting the inclusion criteria were then critically analyzed.

    The following data were extracted from the included studies by using a standardized form: country and region of origin, publication year, sample size, participants' age, penile measurements, population description, and measurement technique.

    3. Assessment of quality for studies included and statistical analysis

    To assess the risk of bias (RoB), each report was reviewed using the National Institutes of Health (NIH) Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [16]. The authors independently assessed the methodological quality based on sequence generation, allocation concealment, enrollment of control groups, incomplete outcome data, selective outcome reporting, and additional sources of bias. Publication bias was tested by visual assessment of the Deeks’ funnel plot [17]. We first obtained the pooled mean and SD for every measurement category (i.e., flaccid, erect, and stretched length). Then, we compared each study measurement with the pooled mean using the standardized mean difference (SMD) and 95% confidence intervals (CIs). Variability in the intervention effects as a consequence of clinical or methodological diversity among the studies was evaluated by form of heterogeneity [18]. Our results are graphically displayed as forest plots, with pooled means and SMD. Evaluation for presence of heterogeneity was done using [19]: (1) Cochran’s Q-test with p<0.05 signifying heterogeneity; (2) Higgins I2 test with inconsistency index (I2)=0%–40%, heterogeneity might not be important; 30%–60%, moderate heterogeneity; 50%–90%, substantial heterogeneity; and 75%–100%, considerable heterogeneity. Subgroup analysis was performed by looking at differences in the regions of origin (i.e., North America, South America, Europe, Africa), population type (volunteers, urology patients, prostate cancer [PCa] patients, others), and the decade of publication (1940–1979, 1980–1989, 1990–1999, 2000–2009, 2010–2021). The QM statistics with accompanying p-values were used to determine the significance of subgroup differences [20]. Sensitivity analyses with and without each study were performed to investigate for any size-effect influences and outlier effects, but no major differences were observed. Metaregression was performed to adjust for preselected covariates (e.g., age, region, patient population) using random-effects models. Statistical tests were performed using RStudio statistical software version 4.2.0 (The R Foundation for Statistical Computing, Vienna, Austria). All tests were two-sided, with a significance level set at <0.05.

    RESULTS

    1. Search results

    The initial search yielded 12,531 articles (PubMed: 1,975; Cochrane: 3,435; and Embase: 7,121). Duplicate articles appearing in multiple databases were excluded (n=8,022). After abstract screening, 7,850 papers were excluded. Of the remaining 172 papers, 97 were further excluded as they either did not report penis measurements (n=63), reported measurements after major pelvic surgeries (n=12), or reported self-measurements (n=22). Full-text articles were then reevaluated and critically analyzed for the remaining 75 articles (Fig. 1). In all, 33, 22, and 64 papers reported data regarding measurements in flaccid, stretched, and erect length, respectively. RoB assessment according to NIH Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies for each of the individual studies is illustrated in Supplement Table 1.

    Fig. 1
    PRISMA flow diagram.

    2. Description of studies

    The study characteristics of each article including patient description and dimensions recorded are summarized in Table 1 [3, 12, 13, 14, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89]. Of the seventy-five studies included, nineteen were conducted in North America [14, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34], nineteen in Europe [3, 12, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51], five in South America [13, 52, 53, 54], eight in Africa [55, 56, 57, 58, 59, 60, 61], twenty in Asia [62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77], one in Oceania [78], and three across multiple regions [79, 80, 81]. Twenty-three studies evaluated volunteers while thirty-six studies reported data from men evaluated for urological reasons. Fourteen studies investigated patients before prostate surgery and two evaluated cadavers. In total, 55,761 men were evaluated. In all, 40,251 (72.1%), 44,300 (79.4%), 18,481 (33.1%) men had data reporting flaccid, stretched, and erect length, respectively. The age ranged from 18 to 86 years with articles published between 1942 and 2021. Among the studies included, fourteen [21, 32, 34, 37, 38, 39, 40, 44, 53, 60, 62, 69, 79, 81] and six [12, 26, 41, 65, 70, 78] reported measurements obtained with penile injections and spontaneous erections, respectively.

    Table 1
    Characteristics of studies assessing penile measuraments

    3. Pooled means and SMD

    Thirty-three studies reported flaccid length with measurements ranging from 5.20 cm to 13.80 cm. The pooled mean estimate under a random-effects model was 8.70 cm (95% CI, 8.16–9.23). Sixty-four studies analyzed stretched penile length with measurements ranging from 8.98 cm to 17.50 cm. The pooled mean estimate under a random-effects model was 12.93 cm (95% CI, 12.48–13.39). Twenty studies analyzing erect length had measurements ranging from 9.50 cm to 16.78 cm. The pooled mean estimate under a random-effects model was 13.93 cm (95% CI, 13.20–14.65). Each study measurement was compared to the pooled mean to reveal the SMD estimate under a random-effects model (-0.05 cm; 95% CI, -0.21 to 0.12). There was evidence of heterogeneity between the studies (Q=2,986.24, df=26, p<0.0001; I2=98.9%). The SMD estimates displayed a temporal trend with more recent studies displaying means higher than the pooled mean (Supplement Fig. 1). Supplement Fig. 2 and 3 reports all SMD for flaccid and stretched length.

    4. Subgroup analyses

    The pooled means and 95% CIs of all the subgroup analyses are summarized in Table 2. Significant differences were noted for geographic region for flaccid (QM=24.19, df=4, p<0.0001), stretched (QM=29.26, df=5, p<0.0001), and erect length (QM=22.86, df=6, p<0.0001). Differences between subject populations were not statistically significant for flaccid (QM=4.16, df=3, p=0.25), stretched (QM=1.12, df=3, p=0.77), and erect length (QM=1.11, df=2, p=0.58). No differences were observed when taking into consideration technique to achieve an erection (QM=2.29, df=1, p=0.13).

    Table 2
    Pooled means and 95% CIs from subgroups analysises investigating decades, regions, and population type

    5. Metaregression analysis

    There was no significant association was found between year of publication and stretched penile length (Fig. 2A). On the contrary, there was a significant association between year of publication and erect penile length (Fig. 2B) which remained significant after adjusting for geographic region, age, technique to achieve erection, and subject population (adjusted estimate: 0.11, p=0.034, Fig. 3). When the same analysis was performed investigating each region singularly, the same trend was observed in studies published in Asia (adjusted estimate: 0.17, p=0.005) and Europe (adjusted estimate: 0.16, p=0.04). Similar trends were also reported when analyzing only urology patients (adjusted estimate: 0.15, p=0.001) and volunteers (adjusted estimate: 0.07, p=0.02). In contrast, age was not associated with penile size: flaccid length (adjusted estimate: 1.84, p=0.079), stretched length (adjusted estimate: 1.93, p=0.372), and erect length (adjusted estimate: 1.41, p=0.505). Using estimates from the metaregression model, erect penile length increased by 24% over the 29 years of observation was observed (from 12.27 cm to 15.23 cm).

    Fig. 2
    Meta-regression model for mean (A) stretched length and (B) erect length over the year of publication.

    Fig. 3
    Meta-regression model for mean erect length over the year of publication by (A) regions of origin, (B) age groups, (C) population type and (D) technique to achieve erection. ICI: intracavernosal injection.

    6. Publication bias

    The funnel plot for three CIs (90%, 95%, and 99% corresponding to shades white, gray and dark gray) for studies presenting flaccid length (Supplement Fig. 4A), stretched length (Supplement Fig. 4B), and erect length (Supplement Fig. 4C). The Egger’s test of asymmetry showed no significance for erect length (Z=0.85, p=0.40) and flaccid length (Z=0.56, p=0.57). On the contrary, there was significant assymmetry for stretched length (Z=2.09, p=0.04).

    DISCUSSION

    The current study identified an increase in the average erect penile length in men from 1992 to 2021. Importantly, the increase was seen across several geographic regions and subject populations. Moreover, when adjusting for relevant covariates, the point estimates remained similar. In contrast, no change was identified in stretched penile length or flaccid penile length. To our knowledge, this is the first study to examine temporal change in penile size. In addition, the current work identified significant differences in penile size measurements across different geographic regions. Moreover, it presents normative penile measurements based on data from more than 55,000 men.

    A temporal trend was noted for erect length but not other penile length measurements. While erect length is fixed, investigators have noted the subjectivity and variability of stretched length. The goal of a stretched penile length measurement is to approximate the penile length during an erection. However, Schneider et al [39] compared younger (18–20 y) and older (48–60 y) men and found that older men had a significantly longer stretched penis, but no difference in erect lengths implying penile elasticity may change with age. Chen et al [37] also measured the forces required to stretch the penis to its full length using a specially developed gauge. In order to reach the erect length, a minimum tension force of 450 g a force during penile stretching is required. When measured, the clinician's force was lower (428 g of force) thus questioning the reliability of this method of measurement. Indeed, the current report noted significant asymmetry in stretched penile lengths suggesting clinical heterogeneity in reported lengths. Moreover, Habous et al [90] reported significant limitations of flaccid and stretched measurements in estimating erect length as well as marked interobserver variation. Thus, estimating penile size in the flaccid state may be inaccurate whether stretched or not.

    While erect lengths are consistent, erect lengths measurements can also create challenges. Different techniques have been described to measure the erect length including self-report, in office spontaneous erection, and in-office intracavernosal (i.e., penile) injection. Because of their inherent biases, self-reported lengths should be regarded with caution. Studies attempting to analyze spontaneous erections in the clinic, on the other hand, have omitted numerous individuals who were unable to “perform” in this unnatural scenario [39]. The simplest technique to achieve an erection is penile injections which are routinely utilized to generate an erection in clinical settings [21, 37, 90]. Importantly, when the current analyses were adjusted for the technique to achieve erection, the point estimates remained similar.

    The current report identified a significant difference in penile measurements across different geographical regions. Geographic variation is consistent with prior reports with other investigators also identifying longer measurements in sub-Saharan Africans, intermediate in Europeans, South Asians, and North Africans, and smaller in East Asians [91]. However, the cause for differences remains unknown and as migration continues, reported variations may lessen with time.

    The etiology of the increase in erect penile length over time remains uncertain. It can be speculated that these changes may be linked with observations that pubertal milestones are occurring in younger boys than in the past [92]. Data suggests that earlier pubertal growth may be associated with increased body sizes including longer penile length [93, 94, 95]. The etiology of temporal changes in puberty remains unknown. Investigators have hypothesized sedentary lifestyle/obesity or increasing exposure to hormone-disrupting substances may play a role [96, 97, 98]. Indeed, emerging data suggest that diverse prenatal or postnatal exposures may influence pubertal timing [99, 100, 101, 102]. Temporal declines in sperm counts and serum testosterone levels, higher rates of testicular tumors, and increasing genital birth defects have also been attributed to environmental and lifestyle exposures [7, 8, 9, 10].

    Certain limitations warrant mention. While measurement techniques were similar across studies, slight variations could contribute to differences. As has been suggested by other studies, the penile measurements may be affected by temperature, arousal state, body size and investigator factors [5, 35, 90]. In addition, volunteer bias may occur in some studies. Importantly, such limitations would be unlikely to consistently change over time to lead to the identified trends. Finally, detailed geographic variation disparities were not taken into consideration in regional analyses because the majority of research did not provide precise information.

    CONCLUSIONS

    Our systematic review and meta-analysis suggest that the average erect penile length increased between 1992 and 2021. Given the important implications of genital development for urinary and reproductive function, future studies should attempt to confirm the trend and identify the etiology.

    Supplementary Materials

    Supplementary materials can be found via https://doi.org/10.5534/wjmh.220203.

    Supplement Table 1

    Risk of bias assessment according to NIH Quality Assessment Tool for Observational Cohort and Cross-Sectional studies

    Click here to view.(118K, pdf)

    Supplement Fig. 1

    Standardized mean difference between pooled mean and measuraments presented in studies reporting erect length. SMD: standardized mean difference, CI: confidence interval, I2: inconsistency.

    Click here to view.(80K, pdf)

    Supplement Fig. 2

    Standardized mean difference between pooled mean and measuraments presented in studies reporting stretched length. SMD: standardized mean difference, CI: confidence interval, I2: inconsistency.

    Click here to view.(83K, pdf)

    Supplement Fig. 3

    Standardized mean difference between pooled mean and measuraments presented in studies reporting flaccid length. SMD: standardized mean difference, CI: confidence interval, I2: inconsistency.

    Click here to view.(79K, pdf)

    Supplement Fig. 4

    Funnel plot presenting 3 levels of confidence interval (90%, 95%, and 99% corresponding to shades white, gray, and dark gray) for (A) flaccid, (B) stretched, and (C) erect length studies.

    Click here to view.(249K, pdf)

    Notes

    Conflict of Interest:The authors have nothing to disclose.

    Funding:None.

    Author Contribution:

    • Conceptualization: FB, ME.

    • Data curation: FB, SB, FDG, FG.

    • Formal analysis: FB.

    • Investigation: FB.

    • Methodology: FBF, ME.

    • Supervision: ME.

    • Validation: ME, AS.

    • Visualization: FB.

    • Writing – original draft: FB.

    • Writing – review & editing: EM, WM, SB, GF, EP, FM, AS, ME.

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    Humans
    Male
    Penis
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