Published in the academic journal Infancy, the study looked at 40 infants aged 3.5 to 5 months, and tested their ability to match infant vocalizations with facial expressions. The infants were seated in front of two video monitors, one displaying a happy, smiling baby and the other a sad, frowning baby. The researchers then piped in audio of a third baby. When the audio was of a happy baby, the infants looked more to the video of the baby with positive facial expressions. When the audio was of a sad, crying baby, the infants looked to the video with negative facial expressions.