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So you're in line at the grocery store, and there's a parent holding a baby right in front of you. You're minding your own business, but when you look up, that baby just won't stop staring at you like you're the most interesting thing on the planet.

You might feel like you've grown a second head, or like you have chocolate smeared all over your mouth, but in reality, that baby just likes looking at your face. Because there's a lot to figure out there! From very early in life, babies seem to like looking at faces, or even just configurations of shapes that look like faces.

They also prefer faces they recognize to ones they don't, but they'll sometimes spend a little more time looking at strangers because they're new or different. Faces are important to babies because they're basically completely helpless and have to rely on their caregivers to survive. They need to be able to find and communicate with the people who feed them and change their diapers, and knowing what they look like is an important part of that.

Even if said communication just involves screaming at the top of their lungs for a while. Babies are also busy taking in huge amounts of information to help them understand the world, and faces can teach them a lot. From our expressions to how our mouths form words, a lot of our communication as humans happens through our faces.

In those first few months of life, babies are not only beginning to learn how to recognize their parents or grandparents, but also about all the emotions conveyed by facial expressions. There's a lot to learn! So when a baby is staring at you in the grocery store, her little brain is probably trying to figure if she knows you and what categories your face fits into, like if you're happy or sad.

Now, babies might be born ready to learn from the world around them, but there's still some question about whether looking at faces is an innate characteristic— one you're born with—or if it's influenced by experience. And, like with a lot of the age-old, nature versus nurture debate, it's a little bit of both. On the nature side, there's some evidence that, within hours to days of being born, babies can copy facial expressions they see, like if you stick out your tongue at them.

Not all studies agree with those results, but it's possible that there's an innate ability for finding and mimicking expressions. Other research has found that within hours of birth, babies can discriminate between their mother's face and a stranger's, which might mean their brain is ready to recognize faces without much information. So, some evidence suggests that babies may be predisposed to look for information from faces right when they're born.

But other studies have shown that it takes experience to get really good at it. Either way, all the time babies and their caregivers spend looking at each other causes rapid improvement in babies' abilities to process and recognize faces. Starting around 3 months old, they even begin to group different kinds of faces— like human versus animals and kids versus adults.

One especially cool study had babies and adults look at a series of different monkey faces, and also at a bunch of different adult human faces. They found that 6-month-old babies are actually better at differentiating between monkey faces than adults are! But when they're about 9 months old, that ability goes away because of what's called perceptual narrowing, where their perception starts to change based on their experience.

Basically, the baby learns that paying attention to the differences in human faces is way more important for its life than the differences in monkey faces, so it should focus more on humans than monkeys. As the brain develops over time, the pathways used to recognize human faces get more efficient, and the ones for monkeys get pruned away. But when researchers expose babies to lots of monkey faces between when they're 6 and 9 months old, they effectively teach their brains that monkeys are important to pay attention to, so they keep that ability for a lot longer.

Even though babies are born with some skills, experience helps tell them what parts of the world they should pay attention to, and that shapes how their brain's perception and recognition systems develop. So, it's a little bit of nature and a little bit of nurture that goes into that cute, big-eyed baby staring at you. And yeah, in that moment, your face might just be the most interesting thing on the planet.

But don't rule out the possibility that you might have a little chocolate on your face, too. Thanks for watching this episode of SciShow Psych, brought to you by our patrons on Patreon. If you'd like to help us keep exploring all the weird and wonderful things that happen in our brains, you can go to patreon.com/scishow. [OUTRO ♪].