"It's critically important, long before you can fend for yourself, to say, 'That's a face that I need to be concerned about. I don't know why, but, at some point, I'm going to know that I should run away from a face like that,'" Nelson said.
The amygdala, an almond-shaped structure deep in the brain, provides the ability to read emotions, and it gets better at it with experience. A baby's brain is wide open to learning -- unlike an adult's brain -- so if a baby doesn't get the kind of exposure to the various expressions, the child's ability to differentiate between facial expressions may not develop as strongly.
"There's actually some evidence to suggest that you can't make up for it," Nelson said.
The skill for reading expressions is valuable. A skilled face-reader will have a better chance at relationships, getting a job and interacting with others, he said.
"Even as adults, we depend heavily on reading faces, and it's because the verbal channel can't always be depended on, because people can disguise that," he added. "They have a harder time disguising their face."
Nelson hopes the research will lead to early diagnoses for autism.
"We hope to see a signature in their brain, or in their eye movements, that will say, 'This is the baby that we're most concerned about,'" he said. "And we can then actually predict who will develop the disorder a year later."
Results from Nelson's experiments also have shown that a child's environment can influence their ability to read faces.
Children who were physically abused at a young age have proven to be much faster at detecting an angry face than a child who was not abused.
Nelson said researchers in Kentucky found that while most infants prefer female faces, babies raised by stay-at-home dads preferred male faces.
But nurture isn't everything. Scientists also have found that genetics may play a role.
For example, scientists recently have discovered that the 5-HTT gene, which codes for the powerful brain chemical serotonin, can determine the risk of depression. Each person has two copies of the gene, called alleles, and each copy can vary in length. Get a short-short combination of 5-HTT and you're at higher risk for depression than if your get a long-long combination.
Nelson thinks 5-HTT also may influence a child's ability to decipher emotions on faces.
"If you have short-short, you show more brain activity for fearful faces than if you had long-long," said Nelson.
Despite all of Nelson's discoveries, he said his lab and others have yet to try testing a child's ability to recognize deceit. According to other scientists, it may be difficult to study lying through facial recognition alone.
"There is no facial indicator of deception. There's nothing that you do with your face when you're lying that you don't do at other times," said Jeff Cohn, a professor of psychology at the University of Pittsburgh.
Cohn works with computer scientists to classify, analyze and detect emotions on people's faces, including pain and clinical depression. Rather than look at still photographs, Cohn said it's easier to detect deceit through the order and intensity of facial movements.
For example, take a person's smile when they receive a gift.