The researchers first analysed the facial structure of 241 competitors in the Ultimate Fighting Championship (UFC). Why choose this competition? "[T]he 'no‐holds‐ barred' nature of the fights and the process of 'cutting' serially defeated combatants from the championship makes for a somewhat Darwinian environment, well‐suited to the investigation of fighting ability," they explained.
The paper reveals that "the width of a man's face [determines] with accuracy his likely fighting ability." Survival of the fittest apparently correlates with having a relatively wide mug, or in scientific lingo 'facial Width-to-Height Ratio' (fWHR). The paper also suggests that a wider face may have evolved as a structural mechanism to be more resistant to punches.
So I did a little more digging on this facial-width indicator, and another study claims that traits such as racism are prevalent among males with broad faces, adding that "fWHR may be a physical manifestation of dominance motives in males and may be best described as an inclination toward interpersonal social dominance and related behaviors."
Both studies cite the presence of high levels of testosterone in males as the culprit. I've posted about the male hormone previously.
As far as facial configurations go as a predictor for fighting prowess or social (or anti-social) dominance, I'm a tad skeptical. I'm reminded of the old wives tale of "criminals have shifty eyes" from my parent's generation. Perhaps more research needs to be done before fighting ability is potentially conflated with negative stereotypes due to face broadness.