Letting a baby 'cry it out' goes against biology

SAITAMA, Japan, April 21 (UPI) -- Parents are often advised to let a baby "cry it out" in a crib but Japanese researchers say to let babies cry and not pick them up goes against biology.

Kumi Kuroda of the RIKEN Brain Science Institute in Saitama, Japan, said there is a very good reason mothers often carry their crying babies to help them calm down -- infants experience an automatic calming reaction upon being carried, whether they are mouse or human babies.

The study showed an infant calming response to carrying is made of a coordinated set of central, motor and cardiac regulations and an evolutionarily conserved component of mother-infant interactions, the researchers said.

"From humans to mice, mammalian infants become calm and relaxed when they are carried by their mother," Kuroda said in a statement.

Kuroda said the idea for the study came while cleaning the cages of her lab's mouse colony.

"When I picked the pups up at the back skin very softly and swiftly as mouse mothers did, they immediately stopped moving and became compact. They appeared relaxed, but not totally floppy, similar to a human baby," Kuroda said.

Once the researchers found an ECG monitor electrodes small enough to use on conscious mouse pups, the researchers found when the baby mice were carried their heart rates slowed, they became calm, relaxed and stopped crying.

The study, published in the journal Current Biology, found the heart rates of human babies slowed immediately upon being carrying -- same as mice.