ScienceDaily (May 7, 2009) - A new study from Northwestern
University shows what many mothers already know: their babies are a
lot smarter than others may realize.
Though only five months old, the study's cuties indicated through
their curious stares that they could differentiate water in a glass
from solid blue material that looked very much like water in a similar
The finding that infants can distinguish between solids and liquids
at such an early age builds upon a growing body of research that
strongly suggests that babies are not blank slates who primarily
depend on others for acquiring knowledge. That's a common assumption
of researchers in the not too distant past.
"Rather, our research shows that babies are amazing little
experimenters with innate knowledge," Susan Hespos said.
"They're collecting data all the time."
Hespos, an assistant professor of psychology at Northwestern, is
lead author of the study, which will appear in the May 2009 issue of Psychological
In a test with one group of infants in the study, a researcher
tilted a glass filled with blue water back and forth to emphasize the
physical characteristics of the substance inside. Another group of
babies looked at a glass filled with a blue solid resembling water,
which also was moved back and forth to demonstrate its physical
Next all the infants were presented with test trials that
alternated between the liquid or solid being transferred between two
According to the well-established looking-time test, babies, like
adults, look significantly longer at something that is new, unexpected
The infants who in their first trials observed the blue water in
the glass looked significantly longer at the blue solid, compared to
the liquid test trials. The longer stares indicated the babies were
having an "Aha!" moment, noticing the solid substance's
difference from the liquid. The infants who in their first trials
observed the blue solid in the glass showed the opposite pattern. They
looked longer at the liquid, compared to the solid test trials.
"As capricious as it may sound, how long a baby looks at
something is a strong indicator of what they know," Hespos said.
"They are looking longer because they detect a change and want to
know what is going on."
The five-month-old infants were able to discriminate a solid from a
similar-looking liquid based on movement cues, or on how the
substances poured or tumbled out of upended glasses.
In a second experiment, the babies also first saw either liquid or
a similar-looking solid in a glass that was tipped back and forth.
This time, both groups of infants next witnessed test trials in which
a cylindrical pipe was lowered into either the liquid-filled glass or
the solid-containing glass.
The outcomes were similar to those of the previous experiment.
Infants who first observed the glass with the liquid looked longer in
the subsequent test when the pipe was lowered onto the solid.
Likewise, the infants who looked at the solid in their first trials
stared longer when later the pipe was lowered into the liquid.
The motion cues led to distinct expectations about whether an
object would pass through or remain on top of the liquid or solid, the
Northwestern researchers noted.
"Together these experiments provide the earliest evidence that
infants have expectations about the physical properties of
liquids," the researchers concluded in the Psychological Science
Hespos primarily is interested in how the brain works, and, to that
end, her research on babies' brand new, relatively uncomplicated
brains provides invaluable insights. She also is doing optical imaging
of babies' brains, in which the biological measures confirm behavioral
"Our research on babies strongly suggests that right from the
beginning babies are active learners," Hespos said. "It
shows that we perceive the world in pretty much the same way from
infancy throughout life, making fine adjustments along the way."
In addition to Hespos, the co-investigators of the Psychological
Science study are Alissa Ferry, a graduate student, and Lance Rips,
professor of psychology, at Northwestern.
Hespos et al. Five-Month-Old Infants Have Different Expectations
for Solids and Liquids. Psychological Science, 2009; 20 (5):
603 DOI: 10.1111/j.1467-9280.2009.02331.x
Adapted from materials provided by Northwestern University.