Newborns laugh in their sleep say researchers from the University of the Sacred Heart and the Primate Research Institute of Kyoto University (both in Japan).

Babies were previously believed not to be able to laugh until they were older than four months, but the research found newborn infants began laughing as early as 17 days after their birth.

The survey\'s findings are thought to indicate that babies begin to develop both emotions and means of expressing those emotions at a much earlier age than previously believed.

The universities plan to release the findings at a symposium held by the Child Research Net Laboratory in Kyoto next Saturday and Sunday.

With parental cooperation, researchers videotaped six babies from four to 87 days old as they slept.

One baby girl laughed out of the corner of her mouth when she was 17 days old and four other babies also laughed one to four times in their first or second month of life, the researchers said.

Babies are known to smile without making sounds immediately after birth, but as it is also believed to be a cramp in the muscles of the face, it is hard to link such smiles to emotion.

What a 17-day-old baby finds funny remains unknown, but Kiyobumi Kawakami, a professor of psychology at University of the Sacred Heart, said that the baby was probably developing emotions.