A new Yale study suggests that humans should thank evolution for our hesitation to reach out and touch poison ivy.

Researchers at the Infant Cognition Center discovered that infants took five seconds longer on average to touch plants than other novel objects, a finding that demonstrated a potential evolutionary origin of the behavior. Researchers said this behavior is an innate defense mechanism against dangers such as toxins or thorns present on the plants. This behavior retains benefits today, since parents can still intervene to prevent children from touching potentially harmful plants, said Annie Wertz, co-author of the study and psychology postdoctoral fellow.

“Plants were a fundamental food source that presented both costs and benefits in their interactions with human beings,” Wertz said. “Although they provide a food source, there is a possibility of coming in contact with gnarly toxins that they must protect themselves against. Since plants present a stagnant danger, it is often better to just avoid interacting with them.”

In the first part of the study, researchers recorded how long it took babies to reach out and touch a series of objects, including real plants, fake plants and novel objects that mimicked certain qualities of plants like the color and shape — like a tube with streaming paper that resembled leaves. To make sure the babies did not grab at the objects that contained certain plant characteristics, they also exposed the subjects to common objects like spoons and lamps.

They found that babies took five seconds longer to touch the real plants than the objects that featured only certain characteristics of the plant, showing that babies hesitated to touch plants as a whole and not simply characteristics of it like its green color or leaves, Wertz said. They also found no difference in the time it took to touch the objects that featured plant characteristics and household objects, demonstrating that babies were not attracted by its novel attributes.

Since babies have limited exposure to plants, the results of the study suggest the demonstrated avoidance is more innate than previously suspected, said Laurie Santos, a Yale professor of psychology, in an email to the News. Santos said she is collaborating with Wertz to study whether infant capuchin monkeys might show similar effects as demonstrated in this study, adding that existing data suggests the effect is not present among adult monkeys.

The information uncovered with infant experiments can be applied to our understanding of how innateness interacts with environmental exposure, said Brian Scholl, a Yale professor of psychology.

“The nature of the mind is not arbitrary — think about the challenges faced by our ancestors versus their modern counterparts. Plants were a major component of how they survived,” he said.

The study opens questions about what part of the plant is key in identifying it as an entity and not its individual components, Wertz said.

The Infant Cognition Center at Yale opened in 1990.