A new study from the University of Western Australia has offered fresh insight into the long-debated question of whether penis size influences female attraction and male perception of rivalry.

The research involved more than 800 participants, including over 600 men and 200 women.

Participants were shown anatomically accurate, computer-generated male figures that varied in height, shoulder-to-hip ratio, and flaccid penis length. Women were asked to rate sexual attractiveness, while men assessed how physically threatening and sexually competitive the figures appeared to them. Some viewed life-sized projections in a laboratory, while others completed an online survey using scaled images.

The findings, published in PLOS Biology, showed that women generally preferred taller men with broader shoulders and a more V-shaped body, with penis size increasing attractiveness only up to a point. Women’s interest decreased beyond around four inches flaccid length, but men kept viewing larger sizes as more threatening.

Dr Upama Aich explained: “Females rated male figures that were taller, had a higher shoulder-to-hip ratio – indicating a more V-shaped body – and a larger penis as being more attractive. However, beyond a certain point, further increases in penis size, height and shoulder breadth had diminishing benefits.”

She added: “Males also rated taller figures that had a more V-shaped body and a larger penis as being more intimidating as sexual rivals and fighting opponents. But, in contrast to female participants, they consistently ranked males with more exaggerated traits as more of a sexual threat, suggesting that males tend to overestimate the importance of these characteristics for attracting females.”

The study also found that height and body shape played a stronger role than penis size in how men judged rivals, although penis size still influenced perceptions of competition.

Individual traits mattered too, with taller women placing more emphasis on male height and older men giving greater weight to penis size when evaluating rivals.

Reflecting on evolutionary implications, Dr Aich noted: “Relative to body size, the human penis is larger than that of other primates, a fact that has puzzled evolutionary biologists. Before the invention of clothing, the penis would have been a prominent feature that might influence potential mates and competitors.”

Co-author Professor Michael Jennions added: “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.”

Overall, the research provides the first experimental evidence that penis size plays a role in both mate selection and male rivalry, while also highlighting a key mismatch: men tend to place more importance on extreme physical traits than women actually do.

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