Mothers have been warned for years that sleeping with their newborn infant is a bad idea, because it increases the risk that the baby might die unexpectedly during the night.

Perhaps it is early nerve development that makes some infant girls show more intelligence than infant boys.

In fact, it's one of the most famous pieces of research about infant emotion ever published.

right now, to reduce the risk of sudden infant death syndrome, the American Academy of Pediatrics recommends that mothers not sleep in the same bed as their babies, but sleep in the same room.