Newswise — 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 the expectations that liquids deform to fill the space allotted and that objects can pass through liquids? Psychologists Susan Hespos, Alissa Ferry, and Lance Rips from Northwestern University wanted to see if infants have different expectations for how solids and liquids behave.
A group of 5-month-old infants were initially presented with either a liquid or a solid. For the liquid condition, the infants saw a glass filled with blue water and the researcher tilted the glass back and forth to show the physical characteristics of the substance inside. Similarly, for the solid condition, a glass filled with a perceptually similar blue solid was moved back and forth to show off its physical properties. Next all the infants were presented with test trials that alternated between the liquid or solid being transferred between two glasses. 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, compared to the liquid test trials. The infants who trained with the solid object showed the opposite pattern — they looked longer at the liquid compared to the solid test trials. 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 the same preliminary trials where the infants saw either liquid or a similar-looking solid inside a glass that tipped back and forth. Next all the infants were presented with test trials that alternated between two events, a cylindrical pipe was lowered into either the liquid-filled glass or the solid-containing glass.
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 provide the earliest evidence that infants have expectations about the physical properties of liquids."
Psychological Science is ranked among the top 10 general psychology journals for impact by the Institute for Scientific Information. For a copy of the article "Five-Month-Old Infants Have Different Expectations for Solids and Liquids" and access to other Psychological Science research findings, please contact Barbara Isanski at 202-293-9300 or [email protected]
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