A new study from the University of Western Australia has shed fresh light on the long-debated question of whether penis size affects female attraction and how men perceive potential rivals.

The research involved more than 800 participants, including over 600 men and 200 women. Participants were shown anatomically accurate, computer-generated images of male bodies that varied in height, shoulder-to-hip ratio, and flaccid penis size.

Women were asked to rate how sexually attractive the figures appeared, while men evaluated how physically threatening and sexually competitive the figures seemed. Some participants viewed life-sized projections in a lab, while others completed an online survey using scaled images.

The findings, published in PLOS Biology, revealed that women generally favored taller men with broader shoulders and a V-shaped body. Penis size did increase attractiveness—but only up to a certain point. Beyond roughly four inches in flaccid length, additional size offered little benefit and could even reduce appeal. Men, however, continued to perceive larger sizes as increasingly threatening.

Lead researcher Dr. Upama Aich explained that while women rated larger physical traits as attractive, the advantages diminished past a certain threshold. Men, on the other hand, consistently viewed exaggerated traits—especially penis size—as signs of stronger sexual competition and physical threat.

She noted that this suggests men tend to overestimate the importance of extreme physical traits when it comes to female preference.

The study also found that height and body shape mattered more than penis size when men judged rivals, though penis size still played a role. Individual differences were important too: taller women placed more emphasis on male height, while older men gave greater weight to penis size when evaluating competitors.

Discussing the evolutionary implications, Dr. Aich pointed out that the human penis is unusually large compared to other primates. Before clothing existed, it would have been a visible feature that may have influenced both mate choice and competition.

Co-author Professor Michael Jennions added that while the penis primarily functions for reproduction, its size likely evolved as a sexual ornament—meant to attract females rather than simply intimidate other males, even though it may do both.

Overall, the study provides the first experimental evidence that penis size influences both attraction and male rivalry, while also highlighting a clear mismatch: men tend to place more importance on extreme physical traits than women actually do.