Even in the home, ABC News' senior health and medical editor Dr. Richard Besser told "Good Morning America" that parents can play games and make faces with their babies to see that recognition in action.
"Does their face change, do they react? If they don't that's something you need to bring to your doctor," he said. "That can be a clue that something is wrong."
In his office, Besser said he often tries to get his littlest patients to turn their heads and follow his facial cues.
"As a pediatrician, when I'm with a mother and a young baby I'm looking to see if there's a connection," he said.
At Charles Nelson's lab at Children's Hospital Boston, babies help him determine when humans begin to recognize faces not by pushing buttons or pointing but simply by looking at pictures on a screen.
Cameras embedded in the screen track the baby's eye movements within a millimeter of accuracy as they examine the faces as a computer logs the child's brain activity.
By comparing changes in eye movement and brain activity for each picture, Nelson can determine when and for how long a child recognizes a difference in a face -- such as in a happy face or a frightened face -- and when a child misses those subtleties.
"There are individual differences to sensitivity to faces, and our hypothesis is they have the origins early at life," said Nelson, a professor of pediatrics and neuroscience at Harvard Medical School.
Nelson said children may start distinguishing faces by species, gender, race and age by their first birthday. However, there's a tradeoff: The more specialized humans become in recognizing some faces, the more they sacrifice their ability to recognize other faces.
"If I showed you [an adult] two different monkeys, you would have a difficult time distinguishing the two faces. You could do it, but it would take time," said Nelson, who has a doctorate in developmental and child psychology.
However, "at 6 months of age, a baby can easily discriminate two monkey faces, but at 9 months, it's difficult for them," he said.
Nelson's work has shown that infants are capable of distinguishing happiness, fear, anger, sadness and disgust even before they can speak. Humans are especially good at recognizing fear.
One baby in the study, 8-month-old Emmett, was shown images depicting three different facial emotions.
He seemed more interested on the one showing fear.
"Babies like to look more at fear, and they show more brain activity to fear," Nelson said. "And we've been puzzled by this. They don't seem ... at all upset or alarmed looking at it, they just look at it more."
He isn't sure why babies pay more attention to fearful faces, but Nelson hypothesizes it is hardwired into children's brains to protect them. The faster a child picks up on mom and dad's fear, the faster the child realizes there's danger.