A popular facial metric for predicting male aggression fails a major test

For years, behavioral scientists theorized that a specific facial proportion signals aggression and dominance in men, but a recent evaluation overturns this idea. By analyzing nearly 2,000 faces, researchers found that this popular metric does not meaningfully differ between men and women, pointing instead to absolute facial width as a better indicator of biological differences. The findings were published in the journal Evolution and Human Behavior.

The human face conveys a wealth of social information, and researchers have spent decades trying to quantify which specific features communicate traits like threat or trustworthiness. One metric that has received massive attention is the facial width-to-height ratio. This is calculated by measuring the horizontal distance between the left and right cheekbones and dividing it by the vertical length of the face.

Past studies suggested that men have a larger facial ratio than women, a physical difference known as sexual dimorphism. Some researchers proposed that this wider-faced proportion evolved as a biological signal of formidability and aggression. However, the evidence supporting this theory has been mixed and heavily debated in recent years.

A primary issue in previous research is the failure to account for allometry, which refers to how physical traits scale with overall body size. Because men are generally taller and heavier than women, a difference in facial proportions might just be a byproduct of being larger overall. Failing to adjust for basic body weight can lead researchers to false conclusions about facial shapes.

Another major point of confusion revolves around exactly how to measure the face. While facial width is consistently measured from cheekbone to cheekbone, researchers have historically defined facial height in various ways. Some measure from the upper lip to the brow, while others measure from the bottom of the chin to the bridge of the nose. These variations in technique have produced conflicting results, making it difficult to establish a scientific consensus.

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To address these issues, lead author Alex L. Jones of Swansea University, alongside researchers Tobias L. Kordsmeyer, Robin S.S. Kramer, Julia Stern, and Lars Penke, designed a study to rigorously evaluate facial proportions. They gathered a collection of 1,949 facial photographs of men and women from various Western societies. All images were taken under controlled laboratory conditions with subjects facing the camera with a neutral expression.

In their first study, the research team used automated software to map 68 distinct points on each face. They calculated five different variations of the facial width-to-height ratio to account for the multiple measurement styles used in historical research. They then evaluated the data using a Bayesian statistical model. Unlike traditional statistical tests that simply look at isolated data points, Bayesian models incorporate prior evidence from the wider scientific literature to calculate the probability of a specific hypothesis.

Importantly, the researchers adjusted their model to account for each participant’s measured physical height and weight. This allowed them to isolate differences in facial shape from general differences in body size. They also established a threshold for what constitutes a meaningful physical difference, basing this boundary on the minor visual distortions caused by basic camera lenses.

The analysis revealed that women actually had slightly larger facial ratios than men. However, this discrepancy was extremely small. The researchers compared the observed facial differences to the minor visual distortions caused by basic camera focal lengths. Because something as simple as changing a camera’s zoom can slightly alter the apparent shape of a face in a photograph, any true biological difference would need to be larger than this margin of error to be considered meaningful.

The sex difference fell entirely within this boundary of visual distortion, leading the researchers to conclude that the variation was basically noise. Ultimately, they found no robust evidence of a difference in facial width-to-height ratios between the sexes.

The team also discovered that the facial ratio is highly sensitive to overall body size. Taller individuals tended to have smaller ratios, while heavier individuals had larger ratios. This suggests that previous studies linking a wide-faced proportion to aggressive behavior might have inadvertently been capturing the behavioral effects of body weight.

In a second study, the researchers abandoned the ratio entirely and focused solely on absolute facial width, measured as the horizontal distance between the cheekbones. By eliminating the height of the face from the equation, the researchers aimed to see if a simpler, direct measurement offered better insights into biological sex differences. Using the same dataset, they ran a new statistical model to see if this single measurement differed between men and women.

As before, they adjusted the data for the participants’ physical height and weight. They also ran a separate analysis on a subgroup of 305 individuals who had previously undergone precise physical measurements using 3D body scanners. This allowed the team to adjust for upper body size, which included metrics like shoulder width, chest girth, and bicep size.

The results showed a vast difference in absolute facial width between the sexes. The models indicated that men have vastly wider faces than women, even after adjusting for general height, weight, and upper body musculature. The statistical models predicted that a man is almost guaranteed to have a wider face than a woman of the exact same body size.

While these results help clarify human facial anatomy, the study relied on photographs of mostly young adults. Facial structures can change throughout a person’s life, meaning these specific measurements might look different in older populations. Ethnic differences in facial structures could also influence these metrics, which were primarily drawn from Western samples.

Readers should also keep in mind that static photographs only capture fixed bone structures, which is just one aspect of human perception. The study cannot account for the dynamic changes to a person’s appearance that occur during live interactions. In real-world environments, social judgments rely heavily on moving facial muscles, emotional expressions, and vocal tones.

Evaluating still photographs might not capture the nuanced ways humans actually perceive threat or dominance in everyday life. Moving forward, shifting scientific focus away from ratios and toward absolute facial width could provide better insights into human evolution. Understanding this straightforward physical trait may eventually clarify how human facial structures relate to social behaviors.

The study, “Updating evidence on facial metrics: A Bayesian perspective on sexual dimorphism in facial width-to-height ratio and bizygomatic width,” was authored by Alex L. Jones, Tobias L. Kordsmeyer, Robin S.S. Kramer, Julia Stern, and Lars Penke.