Five-Month-Old Infants Have Different Expectations for Solids and Liquids

When we knock over a container of pens, not much happens. However, when we knock over a container of cranberry juice, panic can ensue as we try to catch the container before too much juice spills out. These various reactions to our clumsiness are the result of the understanding that solids and liquids have different physical properties and therefore behave very differently. This may seem obvious to us, but when do we develop this discrimination? Previous research has shown that 2-year-olds have different expectations for solids and liquids, but psychologists Susan Hespos, Alissa Ferry, and Lance Rips from Northwestern University wanted to see if children even younger have this skill.

A group of 5-month-old infants were presented  with either a liquid or a solid. For the liquid condition, the infants saw a glass filled with water and the researcher moved the glass around (i.e., tilting to the side) to show the physical characteristics of the substance inside. Similarly, for the solid condition, a glass filled with a plastic object was moved around to show off its physical properties. For the actual experiment, the infants saw as the liquid was transferred into another glass and then watched as the solid transferred into another glass. Infants tend to look longer at new things, so during the experiment, the researchers measured how long the infants looked at the solid and the liquid as they were transferred from one glass to another.

The results, reported in Psychological Science, a journal of the Association for Psychological Science, were very interesting. The infants in the liquid condition group (i.e., those who trained with the liquid in the glass) looked longer at the solid when it was transferred, compared to the liquid when it was transferred. Similarly, the infants who trained with the solid object looked longer at the liquid when it was transferred between two glasses. These results suggest that five-month-old infants are able to discriminate a solid from a similar-looking liquid, based on movement cues—that is, according to how an object moved around in the container, the infants could predict if it will pour or tumble from the glass if it is upended.

The researchers next wanted to test if those results extend to other physical properties that differentiate solids from liquids, such as penetrability. The second experiment had a similar set-up to the first, except instead of seeing the glass moved around, the infants watched as a cylindrical pipe was lowered into either the liquid-filled glass or the solid-containing glass. During the experimental trials, infants watched as this was repeated for both liquid and solid. Just as in the previous experiment, the infants in the liquid condition looked longer when the pipe was lowered onto the solid and the infants in the solid condition stared longer as the pipe was lowered into the liquid. The researchers note this suggests that “motion cues led to distinct expectations about whether an object would pass through or remain on top of the liquid or solid.”

The authors conclude that, “Together these experiments demonstrate that 5-month-old infants are able to use movement cues and solidity to discriminate a liquid from an object of similar appearance, providing the earliest evidence that infants can reason about how liquids behave and interact with objects.”