From elite sprinters to ordinary fruit flies, body symmetry has long been linked to good health, physical strength and reproductive success. But the reasons for that correlation have been difficult to pin down. Is overall body symmetry a marker of genetic fitness? Or does symmetry of specific body parts—eyes, knees or nostrils—confer unique and specific functional advantages? New findings from a long-term study of Jamaican athletes, including some of the fastest sprinters in the world, offer fascinating clues.

In the new work, published November 17 in PLoS ONE, Rutgers University evolutionary biologist Robert Trivers and his colleagues measured the knees, ankles and feet of 73 elite Jamaican track and field athletes, including Olympic runners and one world champion. The researchers also examined the same body areas of 116 nonathlete Jamaicans of comparable age, sex and weight.

The comparison revealed that the sprinters’ knees and ankles were significantly more symmetrical than those joints in the control group. Within the elite sprinter group the knees and ankles of the 100-meter sprinters were the most symmetrical of all; symmetry in the longer-distance sprinters (200, 400 and 800 meters) was less marked but still linked to the best race times. Intriguingly, the feet of the longer-distance sprinters were actually somewhat asymmetrical. Trivers speculates that making frequent left turns in the longer track runs may favor or lead to more foot asymmetry.

Trivers’s Jamaican Symmetry Project has been probing symmetry and genetic fitness for 18 years and he says it keeps turning up novel findings. This latest research further refines the project’s 2013 study of 285 rural Jamaican boys and girls with an average age of eight. For that investigation, Trivers and his colleagues measured subjects’ symmetry from head to toe, including everything from ear length to foot length. The children were measured in 1996, 2006 and a large number were measured again in 2010. Children with symmetric knees in 1996 and 2006 ran faster in 2010. Knee symmetry alone strongly predicted sprinting speed. “In the new study we looked at some of the world’s elite sprinters and, lo and behold, the same variable we isolated in typical, rural Jamaicans stands out,” Trivers says. “Knee symmetry predicts running times.”

These studies offer new insight into the power of symmetry as a predictive trait. The findings make intuitive sense—watch someone run, with legs churning up and down like perfect pistons as they bear down the track—and you can imagine how useful knee symmetry would be. But how much genetic quality is showcased by overall body symmetry, and how much is linked to symmetry of specific body parts? It depends: “Deviation from perfect symmetry occurs during early development,” Trivers explains, “because of stress to the organism and a genetic inability to compensate fully for that stress. A huge scientific literature shows that bilateral symmetry tends to be positively correlated with health, physical strength and speed.”

A colleague on the symmetry project, evolutionary biologist John Manning of the University of Central Lancashire in England, discovered that when racehorses are measured for 10 different body symmetries, symmetry of the head is almost as closely tied to performance as is symmetry of the knees—it’s as if the symmetry of the sensory organs and the brain may be as important as the running apparatus.

Likewise, in humans the degree of nostril symmetry predicts middle-distance running ability, perhaps because such nostril attributes enhance oxygen intake. Foot symmetry predicts physical aggressiveness in boys, possibly because physical stability helps a fighter. Ear symmetry predicts a woman’s tendency to cradle a baby or doll on the left side of her body. One hypothesis for the advantage here is that, once again, symmetry signifies overall fitness and health. Holding an infant on the left side may calm it with the steady thuds of the mother’s heartbeat.

Finally, breast asymmetry is linked to a higher risk of breast cancer—a robust correlation proved through direct measurements of the breasts as well as with x-rays. Here, Trivers says, “asymmetry itself may signal additional abnormalities, such as impaired ability to control the growth of cancer cells.”

But it turns out we don’t need calipers and x-ray machines to tell us how significant symmetry is: Humans are already highly attuned to its importance, and not just in potential mates. For example, a 2009 study by Trivers concluded that generosity depends on our perception of another’s symmetry as well as on our motivation for giving. The research used what’s known as the ultimatum game to measure response to symmetry. In this game a “proposer” offers to split a sum of money with a “responder” who may accept or reject the offer. If the offer is accepted, each receives money; if rejected, nobody receives anything.

In a sample of 106 male and 82 female Jamaicans, individuals played games with responders of the opposite sex in which they were presented a photo to represent the responder. In the first game they received a photo of a symmetrical face; in the second an asymmetrical face. Thirty-five people offered more to the responder because they thought the photo was attractive and of these, 29 picked the symmetrical photo. Eleven people offered more to the responder because they felt needed the responder needed the money. These charitable proposers all chose the asymmetrical photo. The findings suggest a deeply intuitive human knowledge that asymmetrical individuals are, on average, genetically less fit—and therefore disadvantaged.

Perhaps the greatest secret of symmetry has yet to be revealed: how it might subtly enhance human intelligence. There, Trivers suspects, symmetry may be more than the sum of its parts. “Think about ears and eyes,” he says. “We have two symmetrical eyes with overlapping fields of vision, and with that information our brain creates three-dimensional sight.” Similarly, Trivers adds, with two symmetrical ears our brain can synthesize stereophonic sound. “And then there are the two symmetrical lobes of the brain. Does brain symmetry lead to a leap in social intelligence that is the equivalent of three-dimensional vision and stereophonic hearing?”

Manning thinks we may know why, at least in biology, nature so loves symmetry. “Perfect symmetry is difficult to develop and maintain during rapid growth and development,” he says. “The struggle to reach adulthood in symmetrical perfection represents a developmental health test involving many genes. That is why body symmetries reflect everything from enhanced sperm production and faster running speed to predisposition to some cancers.” Symmetry may thus be a true mark of fitness—whether it’s the perfect patella, the nicest nostrils or the most brilliant brain.