How did we come to learn everything we now know about infants and their developing minds? I mean, infants aren’t exactly going to fill out a questionnaire or talk to you about how they’re feeling. The answer? Their eyes. Specifically, how long they look at something.

Eye-tracking research

Researchers use a method called eye-tracking to determine where babies are looking, what they’re looking at, and how long they look at it for.

We rely on the knowledge that an infant is going to look longer at something novel than something he’s used to. When something surprises him or operates in an unexpected way, an infant will stare at it longer.

This is no different to adults; we don’t give the world much of a second thought until it does something that surprises us. Imagine how much longer you would watch a plane zigzagging across the sky than a plane flying in the straight line you’ve come to expect.

Scientists use habituation, a process of getting infants used to seeing something a certain way, to determine whether the infant notices when something changes. When something defies the infant’s expectations and understanding, her looking time lengthens.

Gravity, probability, and more

We’ve come to believe, through decades of eye-tracking studies, that infants are much more aware than they’re typically given credit for, and that they understand seemingly complex concepts like gravity and mathematics.

Yup, that’s right, research has determined that infants understand basic laws of gravity.

Infants in these studies stared longer at objects that defied gravity compared to consistent, expected gravitational movement.

Basic summary: infants were confused when a ball accelerated unnaturally up a hill and stared at it for longer than a ball rolling downhill with a normal rate of acceleration.

Studies have also shown that infants comprehend how likely an outcome is to happen. In other words they also get basic probability.

Researchers in one study showed babies a bag full of pink and yellow balls. Six-month-olds looked longer when a pink ball was drawn out of a bag that held a 4:1 ratio of yellow balls over pink balls. This led the researchers to believe that the infants knew it was more likely that a yellow ball would be drawn out of the bag and they were surprised when it wound up being pink.

Eye-tracking isn’t the only form of cognitive research in infants, but it’s arguably been one of the most pivotal. In many cases, it’s allowed researchers to answer the age-old question of parents everywhere: What the heck is going on in my baby’s head?