New research suggests marmoset monkeys self-domesticated, just like humans, adopting physical changes to match their sociability.

According to scientists, it's not a coincidence that dogs are cuter than wolves and goats at the petting zoo appear cuddlier than their relatives in the wild. Those goats at the state fair don't just act friendlier, they look the part, too -- with shorter horns and floppier ears.

The "domestication syndrome" theory suggests that as aggression is bred out of animal lineages, the animals make physical transformations, adopting shorter muzzles and snouts, curlier tails and paler fur, among other attributes.

Similarly, scientists have long theorized that humans self-selected for pro-social attributes. By opting for less aggressive and more cooperative mates, humans, in essence, self-domesticated.

"The evidence for this has been largely circumstantial," Asif Ghazanfar, a professor of psychology and neuroscience at Princeton University, said in a news release. "It's really a popular and exciting idea but one that lacks direct evidence, a link between friendly behavior and other features of domestication."

In their quest to lend domestication syndrome some scientific legitimacy, researchers turned to the marmoset monkey, a highly social and cooperative species with physical attributes consistent with domestication syndrome. Most prominently, marmosets sport a white tuft of hair on their forehead, a feature common among domesticated animals.

For the study, published Thursday in the journal Current Biology, researchers analyzed how often different marmosets participated in friendly vocal exchanges. They found the friendliest marmosets sported the largest tufts of white hair.

To confirm the causal link, researchers engaged twin marmoset infants with vocal feedback from a computer simulated parent. For one twin, the simulation responded to 100 percent of the infant's vocalizations, but for the other, the simulated parent responded to only 10 percent.

For nearly two months, researchers repeated the simulated engagement every other day for 40 minutes. The infants still spent the vast majority of their time with their families.

The infants that received more engagement not only learned to speak faster, but also grew more white hair on their heads.

The experiment's results suggest domestication syndrome may be closely linked with vocal development.

"If you change the rate of the marmosets' vocal development, then you change the rate of fur coloration," said Ghazanfar. "It's both a fascinating and strange set of results!"

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