An instinct hardwired by evolution causes crying babies to calm down when transported in their mothers' arms, research suggests.
Human infants respond to being carried in much the same way as mice, scientists found.
In both, heart rates slow down and the body becomes super-relaxed. But the same reaction does not occur when an infant is merely held still. It appears to be driven by sensing movement, the research showed.
The findings, published in the journal Current Biology, could help parents who have trouble pacifying their babies, claim the scientists.
"This infant response reduces the maternal burden of carrying and is beneficial for both the mother and the infant," said lead scientist Dr Kumi Kuroda, from the Riken Brain Science Institute in Japan.
The researchers believe the calming response is a survival trait preserved by evolution in many mammals, including lions and squirrels.
The research involved measuring electrical brain activity in both human babies and mouse pups. When infant mice are carried by their mothers, they adopt a characteristic compact posture with limbs flexed and stop emitting high pitched squeaks. In mice, the calming response was dependent on the sense of touch and the ability to perceive body movement.
The findings may have important implications for parenting, say the scientists.
"Such proper understanding of infants would reduce the frustration of parents and be beneficial, because unsoothable crying is a major risk for child abuse," said Dr Kuroda.
The study authors wrote: "Although our study was done on mothers, we believe that this is not specific to mums and can be used by any primary caregiver."