Have you ever wondered why humans differ so dramatically from our closest primate relatives when it comes to genital size? It's not a topic you'll find in polite dinner conversation, but it's a genuine scientific puzzle that has stumped evolutionary biologists for decades. The human penis is disproportionately large compared to body size when you stack us up against gorillas, chimpanzees, or bonobos. What evolutionary pressures could have driven this anatomical quirk?
A groundbreaking study published on January 21, 2026, in PLOS Biology finally offers experimental evidence to answer this question. Researchers from four Australian universities discovered that penis size serves a dual evolutionary purpose: it attracts potential mates and intimidates male rivals. This isn't just speculation or cultural bias—it's backed by data from over 800 participants who rated computer-generated male figures.
We're diving deep into this research to unpack what it means for our understanding of human evolution. Whether you're curious about evolutionary biology, interested in sexual selection, or simply want to understand the science behind human anatomy, this article will give you the full story.
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Table of Contents
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- What Makes Human Anatomy So Different From Other Primates?
- How Was This Study Conducted?
- What Did Women Find Most Attractive?
- How Did Men Assess Their Rivals?
- Which Evolutionary Force Is Stronger: Female Choice or Male Competition?
- What Role Do Height and Body Shape Play?
- What Are the Limitations of This Research?
- What Does This Mean for Our Understanding of Human Evolution?
What Makes Human Anatomy So Different From Other Primates?
When you compare human males to other great apes, the difference is striking. Gorillas, despite their massive size and strength, have remarkably small penises—typically around 3 cm (1.2 inches) when erect. Chimpanzees, our closest genetic relatives, aren't much bigger. Humans? We're sporting an average erect length of about 13 cm (5.1 inches), with flaccid sizes ranging from 5 to 13 cm in this study. psypost
This size difference has puzzled scientists for years. After all, if the primary biological function of the penis is sperm transfer, why would evolution favor something so much larger than what's needed for reproduction ? The answer, it turns out, lies in sexual selection—the process where certain traits evolve not because they help with survival, but because they help with mating success. theconversation
Dr. Upama Aich, a Forrest Research Fellow at the University of Western Australia's Centre for Evolutionary Biology, explains: "The human penis is unusually large compared to our closest primate relatives, which has long been an evolutionary puzzle". Previous research suggested female preference might play a role, but nobody had tested whether males also used penis size to evaluate competitors.
How Was This Study Conducted?
The researchers designed an elegant experiment to isolate specific physical traits. They created 343 anatomically accurate, computer-generated 3D male figures that varied independently in three characteristics: au.news.yahoo
- Penis size: ranging from 5 to 13 cm (flaccid length)
- Height: ranging from 1.63 to 1.87 meters
- Body shape: measured by shoulder-to-hip ratio, ranging from 1.13 (pear-shaped) to 1.45 (V-shaped)
Each trait had seven possible values spanning a natural range found in real human populations. The researchers made sure that longer penises were also proportionally wider, with a 1.2 cm increase in width between the smallest and largest. journals.plos
More than 800 participants viewed these figures either as life-sized projections in person or on personal devices through an online survey. Female participants rated each figure for sexual attractiveness, while male participants assessed how physically threatening and sexually competitive each figure appeared as a potential rival. uwa.edu
The consistency across both viewing methods was remarkable. Whether people saw life-sized projections or smaller images on screens, the ratings yielded comparable conclusions. This suggests the effects are robust and not dependent on specific viewing conditions. theconversation
What Did Women Find Most Attractive?
Women's ratings revealed a clear pattern: taller men with V-shaped torsos and larger penises scored higher on attractiveness. But here's where it gets interesting—the benefits weren't unlimited. journals.plos
For female participants, there was a pattern of diminishing returns. After a certain point, further increases in penis size, height, or shoulder-to-hip ratio offered progressively smaller boosts to attractiveness. Think of it like adding sugar to coffee: the first spoonful makes a big difference, but by the fifth spoonful, you're barely noticing any change. theconversation
The research team used evolutionary selection analysis to measure the strength of these preferences. They discovered that while body shape was the strongest driver of male attractiveness to females, penis size and height had remarkably similar effects. In the online survey, penis size actually had a stronger effect than height. journals.plos
Women also made their judgments quickly. The researchers measured response times and found that participants rated figures with smaller penises, shorter height, and less V-shaped bodies significantly faster. This suggests these traits are processed subconsciously and dismissed quickly when they don't meet certain thresholds. uwa.edu
How Did Men Assess Their Rivals?
Male participants showed a different pattern. When evaluating potential rivals, men consistently ranked figures with more exaggerated traits—larger penises, greater height, and more V-shaped bodies—as stronger competitors and more physically threatening. uwa.edu
Crucially, men didn't show the same diminishing returns that women did. They kept rating bigger as better, suggesting men might overestimate how appealing these traits are to women. This disconnect between male perception and female preference is fascinating from an evolutionary perspective. journals.plos
"Men rated rivals with a larger penis as being more physically threatening and sexually competitive," Dr. Aich explained. This finding provides the first experimental evidence that males consider penis size when assessing a rival's fighting ability and attractiveness. phys
The study found significant correlational selection for all trait combinations. This means the traits interact with each other—a larger penis increased perceived threat more strongly when combined with greater height or a more V-shaped body. journals.plos
Which Evolutionary Force Is Stronger: Female Choice or Male Competition?
Both female mate choice and male-male competition favor larger penis size, but they're not equally strong. The researchers calculated that the effect of penis size on attractiveness to women was four to seven times greater than its effect as a signal of fighting ability to men. eurekalert
This finding suggests that female preference has been the primary evolutionary driver of increased penis size in humans. Male-male competition likely contributed, but it played a supporting role rather than the lead. uwa.edu
Emeritus Professor Michael D. Jennions of the Australian National University, a co-author of the study, summarized it this way: "While the human penis functions primarily to transfer sperm, our result suggests its unusually large size evolved as a sexual ornament to attract females rather than purely as a badge of status to scare males, although it does both". journals.plos
The research supports what's called the "dual signal hypothesis"—the idea that the human penis serves both as a sexual ornament for attracting mates and as a social signal for intimidating rivals. However, the ornament function appears to be the stronger force. psypost
What Role Do Height and Body Shape Play?
Penis size doesn't operate in isolation. The study found significant interactions between penis size, height, and body shape in determining attractiveness. uwa.edu
For women rating attractiveness in the in-person survey, a larger penis elevated attractiveness more strongly for taller men. Similarly, for both survey types, penis size provided a greater benefit to men with a more V-shaped body (greater shoulder-to-hip ratio). journals.plos
The practical implication? A larger penis provided a weaker benefit to smaller, more pear-shaped men. The traits work synergistically—they enhance each other's effects rather than simply adding together. uwa.edu
For male assessments of rivals, the pattern was similar but with some differences. There was a significant interaction between body shape and height, and between body shape and penis size. However, unlike female ratings, the interaction between penis size and height wasn't significant for male perceptions of sexual competitiveness. journals.plos
This previous research from 2013 by some of the same authors had already shown that penis size interacts with body shape and height, but this new 2026 study confirms and extends those findings while adding the crucial male-male competition component. pnas
What Are the Limitations of This Research?
The researchers were transparent about their study's boundaries. They isolated specific physical traits in a controlled environment, but real-world attraction is far more complex.
"Our study used animated 3D models to isolate specific effects of male penis size, height, body-shape," Dr. Aich noted. "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".
The study focused on first impressions based on visual cues alone. It didn't account for:
- Facial features
- Voice and communication style
- Personality traits
- Social status
- Emotional intelligence
- Cultural context
Another important caveat: "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". The study explains why human penises are larger than functional necessity would require, but reproduction remains the primary biological function. psypost
The participants were also primarily from Australian universities, which might introduce cultural biases. Cross-cultural studies would help determine if these preferences are universal or culturally influenced.
What Does This Mean for Our Understanding of Human Evolution?
This research fills a significant gap in our understanding of human sexual selection. While scientists had suspected that female choice played a role in the evolution of larger penis size, this study provides the first experimental evidence that male-male competition also contributed. phys
The findings show that sexual selection—both through female mate choice and male-male competition—has acted on multiple male traits simultaneously. Height, body shape, and penis size combine to determine male attractiveness and perceived fighting ability. journals.plos
Dr. Aich emphasized the complexity: "Our study highlights how complex human social and sexual perceptions are. 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 research demonstrates that precopulatory sexual selection (mate choice before mating occurs) can play a role in the evolution of genital traits. This challenges older assumptions that genital evolution is driven primarily by postcopulatory competition (what happens after mating, like sperm competition).
What we're seeing is the result of thousands of generations of human evolution, where both female preferences and male competition shaped not just our genitalia, but also our overall body proportions. The human body is, in many ways, a walking billboard of sexual selection.
Final Thoughts
The evolution of the human penis is a testament to how sexual selection shapes our species. This 2026 study from Australian researchers provides compelling evidence that both female attraction and male competition have driven the development of larger penis size in humans, with female preference being the dominant force.
But let's not reduce this to a simple "bigger is better" narrative. The research shows diminishing returns for female attraction, complex interactions between traits, and the importance of context. Real-world attraction involves personality, emotional connection, and countless other factors that no computer model can capture.
What makes this research valuable isn't just solving an anatomical puzzle—it's showing us how evolution works in complex, sometimes surprising ways. Sexual selection isn't simple or straightforward. It's a dance between multiple pressures, preferences, and trade-offs that plays out over millennia.
We hope this deep dive has satisfied your curiosity about one of evolution's more intriguing questions. Science isn't always about galaxies and black holes—sometimes it's about understanding ourselves and the evolutionary forces that made us who we are. Keep questioning, keep learning, and remember: knowledge is the best weapon against ignorance.
Visit FreeAstroScience.com for more articles that make science simple, engaging, and accessible. We're here to keep your mind sharp and your curiosity alive.
Sources
Aich, U., et al. (2026). "Experimental evidence that penis size, height, and body shape influence assessment of male sexual attractiveness and fighting ability in humans." PLOS Biology. Published January 21, 2026. https://journals.plos.org/plosbiology/article?id=10.1371%2Fjournal.pbio.3003595
Phys.org. (2026). "Human penis size influences female attraction and male rival assessment." January 21, 2026. https://phys.org/news/2026-01-human-penis-size-female-male.html
EurekAlert! (2026). "Human penis size influences female attraction and male rival assessment." January 21, 2026. https://www.eurekalert.org/news-releases/1112786
Aich, U. & Jennions, M. (2026). "Why are human penises so large? New evolutionary study finds two main reasons." The Conversation. January 22, 2026. https://theconversation.com/why-are-human-penises-so-large-new-evolutionary-study-finds-two-main-reasons-273365
University of Western Australia. (2026). "Evolution of the human penis: why size does matter." January 22, 2026. https://www.uwa.edu.au/news/article/2026/january/evolution-of-the-human-penis-why-size-does-matter
Yahoo! News Australia. (2026). "Why are human penises so large? New evolutionary study finds two main reasons." January 22, 2026.
Mautz, B.S., et al. (2013). "Penis size interacts with body shape and height to influence male attractiveness." Proceedings of the National Academy of Sciences, 110(17). https://www.pnas.org/doi/10.1073/pnas.1219361110
404 Media. (2026). "Scientists Got Men to Rate Penises by How Intimidating They Are." January 21, 2026. https://www.404media.co/scientists-got-men-to-rate-penises-by-how-intimidating-they-are-this-is-what-they-found/
PsyPost. (2026). "Human penis size is an evolutionary outlier, and scientists are finding new clues as to why." January 21, 2026. https://www.psypost.org/human-penis-size-is-an-evolutionary-outlier-and-scientists-are-finding-new-clues-as-to-why/
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