This study, after controlling for various confounding variables, has conducted partial correlation and regression analyses exploring the interplay between penile dimensions (length and girth) and IQ. The preliminary findings are as follows:
There is a notable racial variation in both adult IQ and penile length. In terms of IQ, the ranking is as follows: Mongoloids > Caucasoids > Mixed Ethnicity > Amerindians > Negroids. This order inversely correlates with penile length, with Mongoloids exhibiting the shortest length but the highest IQ, and Negroids having longer lengths but the lowest IQ.
In a flaccid state, controlling for consistent girth, penile length negatively correlates with IQ (P < 0.001). Specifically, a shorter penile length is associated with a higher IQ, showing significant racial disparities (refer to Table 3 Models 1 and 3, and Table 4 Models 1 and 3). This pattern remains consistent even when controlling for flaccid circumference, fully erect length, and circumference. However, in the fully erect state, the correlation between penile length and IQ reverses: a longer erect length corresponds to a higher IQ when girth is constant (P < 0.001) (see Table 3 Models 2 and 3, and Table 4 Models 2 and 3). A comparison of standardized coefficients (Table 4 Model 3) indicates that the regression coefficient for erect penile length (0.07) is lower than the absolute value for flaccid length (-0.08, P < 0.001), suggesting a stronger correlation between flaccid length and IQ.
The apparent contradiction in the relationship between IQ and penile length in its flaccid and erect states may stem from variations in the penile length growth coefficient. Penises with shorter flaccid lengths tend to exhibit higher growth coefficients compared to those with longer flaccid lengths. However, the disparity in circumference expansion coefficients between penises with smaller versus larger flaccid circumferences is marginal.[16–18]
Therefore, for men with equivalent flaccid girth, full erection length, and circumference, those with shorter flaccid lengths have higher IQs, and this trend is markedly evident across different racial groups. Mongoloids, having the shortest flaccid lengths, display the highest IQs, while Caucasoids, Amerindians, Mixed Ethnicity, and Negroids, with longer lengths, have correspondingly lower IQs. These conclusions are further corroborated by subsequent robustness tests involving length growth and circumference expansion coefficients. It is crucial to note, however, that these findings do not imply a causal relationship between penile length and IQ.
The potential influences on the observed correlation between penile length and IQ are multifaceted, encompassing not only genetic, racial, and evolutionary factors but also encompassing a spectrum of social and environmental elements. These include education, cultural norms, dietary habits, exposure to chemicals or environmental pollutants, and climatic conditions. [19–22] Some studies propose a possible connection between penile length and reproductive fitness, suggesting that shorter penile length might correlate with a lower competitive edge in fertility. In scenarios of sperm competition, a longer penis could facilitate the positioning of a male's semen closer to the cervix, potentially enhancing the propagation of his genetic material. This hypothesis is reflected in the reproductive strategies of different racial groups, with individuals of African descent often exhibiting an r-strategy, characterized by a high frequency of sexual activity and a larger number of offspring. In contrast, Mongoloid populations typically demonstrate a K-strategy, focusing on rearing fewer offspring with greater parental investment. [4][23][24]
Genetic differences between races, along with varying levels of androgens, significantly contribute to the regulation of penile size. Notably, the AR gene on the X chromosome (Xq11-12) and the SRY gene on the Y chromosome are implicated in determining penile dimensions. The development of the male penis is also influenced by the secretion of pituitary growth hormone or gonadotropins, while androgens, such as testosterone, are critical during puberty for penis enlargement and elongation.[25] A deficiency in growth hormone (GH) and insulin-like growth factor 1 (IGF-1) can result in smaller penile size.[26]
While this study indicates a negative correlation between penile length and IQ, it is not without its limitations. Firstly, the analysis, although based on data from 139 countries, lacks individual-level microdata within each nation, potentially leading to ecological fallacy. Secondly, the hypothesized inverse relationship between penile length and IQ is yet to be underpinned by extensive theoretical support, warranting further exploration. Finally, the standardized measurements of penile size used in the study may be influenced by various factors such as ambient temperature, arousal state, presence of others, concerns about privacy, or time elapsed since the last ejaculation. These factors could introduce biases in the reliability and validity of the measurements. Additionally, the regression analysis does not account for a range of confounding factors and does not adequately address sample selection bias or possible measurement errors, thus precluding any causal interpretations.