Human penis size is an evolutionary outlier, and scientists are finding new clues as to why

A new study published in PLOS Biology indicates that the size of a human penis influences how men are assessed by others, impacting both their perceived attractiveness to females and their perceived fighting ability among other males. The findings suggest that the unusually large size of the human penis, compared to other primates, likely evolved through a combination of female mate choice and male-male competition. This research offers the first experimental data showing that men use penis size as a visual signal to estimate the threat level of a potential rival.

The human penis is considered an evolutionary outlier because it is significantly longer and thicker than that of other great apes, relative to body size. Humans also lack a penis bone, or baculum, meaning the organ relies entirely on blood flow to maintain rigidity. Biologists have debated the origins of these unique traits for decades.

Some previous theories focused on post-copulatory factors. These hypotheses suggested that the size and shape of the penis might help remove the sperm of rival males or increase the likelihood of fertilization. However, less attention has been paid to pre-copulatory selection, which involves traits that influence the decision to mate or the outcome of physical contests before mating occurs.

In many animal species, physical traits act as signals of dominance or genetic quality. The researchers behind the current study sought to test if the human penis functions as a badge of status or an ornament of attraction. They aimed to isolate penis size from other physical characteristics to see if it independently affects how a male is perceived.

“The human penis is unusually large compared to our closest primate relatives, which has long been an evolutionary puzzle,” said study author Upama Aich, a Forrest Research Fellow at the Centre for Evolutionary Biology at The University of Western Australia. “While previous research focused on how penis size affects female mate choice, we wanted to address the other half of the story: male-male or intrasexual competition. We wanted to test if the penis size plays any role in how men use this trait to assess the fighting ability and sexual competitiveness of their rivals.”

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To conduct this investigation, the research team employed an experimental design. They generated 343 unique male figures using computer software capable of creating anatomically accurate human bodies. These figures varied independently across three specific traits: height, body shape (measured by the shoulder-to-hip ratio), and flaccid penis size.

The researchers ensured a comprehensive range of body types by using seven different values for each of the three traits. The variations in height and penis size covered the natural range typically found in human populations. The body shape ranged from a pear-shaped torso to a broad-shouldered, V-shaped physique.

To allow participants to fully assess the physique, the figures were presented as short video clips. In these videos, the 3D model rotated 30 degrees to the left and right. This rotation allowed viewers to gauge the depth and proportions of the torso and genitals from multiple angles.

The study recruited a sample of over 800 adult participants. This group included approximately 200 females and 600 males. The researchers divided the experiment into two distinct viewing formats to test if the method of presentation influenced the results.

In one format, participants attended an in-person session where the figures were projected onto a wall at life-size scale. This setting provided a realistic representation of height and physical presence. In the second format, participants viewed scaled-down versions of the videos on personal computer screens or tablets via an online survey.

Female participants were asked to rate the figures based on sexual attractiveness. They used a standard numerical scale to record their preferences. Male participants were assigned to one of two different assessment tasks.

One group of men rated the figures based on perceived sexual competitiveness. They were asked to estimate how jealous they would feel if the figure was speaking to their partner. A second group of men assessed fighting ability by rating how threatened they would feel if the figure wanted to start a fight with them.

The analysis of the data revealed consistent patterns across both the in-person and online surveys. “We were surprised by how remarkably consistent our findings were across different contexts,” Aich told PsyPost. “Whether participants viewed life-sized projections in-person, or looked at small screens online, the patterns of assessment were in similar direction. This suggests that these physical assessments might be robust across individuals and different viewing environments.”

Female participants rated men who were taller, had a more V-shaped torso, and possessed a larger penis as more attractive. The study found that these three traits did not work in isolation but influenced one another.

For example, the positive effect of penis size on attractiveness was stronger for men who were already tall or had a V-shaped body. A larger penis provided a smaller boost in attractiveness score to figures that were shorter or had a more pear-shaped torso. This indicates that females assess these physical traits as a composite package rather than individually.

The female ratings also demonstrated a pattern of diminishing returns. While attractiveness scores increased with height, body shape, and penis size, the benefits began to plateau at the higher end of the spectrum. Extreme exaggeration of these traits did not result in a proportional increase in attractiveness rating.

“While penis size is a significant factor in how men and women assess a male’s sexual attractiveness and fighting ability, it doesn’t work in isolation,” Aich explained. “In our analysis, penis size interacted with body shape and height to affect male attractiveness. A larger penis provided a weaker benefit to smaller, more pear-shaped men. Meaning, height and body shape also played a significant role in these assessments.”

Male participants assessing the sexual competitiveness of rivals produced ratings that largely mirrored the female preferences. Men rated rivals with larger penises, greater height, and broader shoulders as more likely to cause jealousy. However, the male ratings differed in the shape of the selection curve.

Unlike the females, male participants did not show a pattern of diminishing returns. Men perceived that the attractiveness of a rival continued to increase linearly with the size of the traits. This finding implies that men may overestimate the extent to which women prefer exaggerated physical features.

The study provided novel insights into how men assess the fighting ability of rivals. Male participants rated figures with larger penises as more threatening and dominant. While height and body shape were the strongest predictors of perceived fighting ability, penis size had a distinct and statistically significant effect.

This suggests that men use genital size as a visual cue when assessing the strength or aggression of a potential opponent. The authors propose that this link might exist because testosterone exposure affects both genital development and muscle mass. A larger penis might therefore be perceived as an honest signal of hormonal fitness and physical capability.

Another potential explanation offered by the researchers involves the physiological response to stress. In humans, the flaccid penis can retract or shrink due to adrenaline release during situations of fear or anxiety. A larger flaccid penis might act as a signal that the rival is calm, confident, and not experiencing a stress response.

The researchers also measured the response time for each rating. They found that participants made decisions faster when viewing figures with a smaller penis, shorter height, and less V-shaped body. The rapid dismissal of these figures suggests that these traits are processed and categorized very quickly by the brain.

“Our study highlights how complex human social and sexual perceptions are,” Aich said. “Rather than focusing on one feature, people integrate multiple cues very quickly, often without conscious awareness. Understanding that complexity would help move discussions away from myths and toward evidence.”

“The main takeaway is that the human penis, while primarily functions for sperm transfer, also acts as a dual signal. It plays a role as a sexual ornament to attract mates and as a social signal to intimidate rivals. Importantly, we found that a larger penis increased male attractiveness and signalled perceived fighting ability.”

There are some limitations. The research utilized figures with flaccid penises, which is the most common state observed in daily life. However, sexual selection and mate choice in primates often involve erect displays, which might elicit different responses or stronger preferences.

Additionally, the study isolated specific physical traits in a controlled, artificial environment. In real-world interactions, assessment of a mate or rival involves a complex integration of cues. Factors such as facial symmetry, vocal pitch, personality, and social status likely interact with body morphology to determine overall attractiveness or dominance.

“A key caution is that the primary evolutionary function of the penis is sperm transfer; note that the signalling roles we studied and detected are secondary and helps explain the larger penis size we see in humans,” Aich said. “Also, our study used animated 3D models to isolate specific effects of male penis size, height, body-shape. In the real world, characteristics like personality, voice, and facial features can play massive roles in how we perceive others. A large penis doesn’t ‘guarantee’ attractiveness or dominance on its own.”

“Future work could examine how these visual cues interact with others, such as facial features, voice, or movement, and whether similar patterns hold across cultures.”

The study, “Experimental evidence that penis size, height, and body shape influence assessment of male sexual attractiveness and fighting ability in humans,” was authored by Upama Aich, Chloe Tan, Rebecca Bathgate, Khandis R. Blake, Robert C. S. Capp, Jacob C. Kuek, Bob B. M. Wong, Brian S. Mautz, and Michael D. Jennions.