The discovery of a gene that controls the bonding process between infants and their mothers promises new insights into autism and other behavioural disorders.

An experiment in Italy has shown that knocking out a single gene transforms the way in which newborn mice relate to their mothers.

When mouse pups are separated from their mothers, they normally handle the experience badly, becoming distressed and screaming in high-pitched wails that are the equivalent of a human baby's crying.

However, infant mice that lack a particular gene, an opioid receptor, behave very differently. When separated from their mothers they do not seem to care at all. While their normal cousins scream to high heaven, the mutant pups sit quietly and go to sleep. The mutants also showed no preference for their mother's smell over other mice.

Apart from their cold attitude towards their mothers, the mutant mice were normal in every way.

The findings, by a team led by Anna Moles, of the Italian National Council for Research in Rome, indicate that the receptor plays a critical role in bonding between infants and their mothers. This receptor is the same one that responds to opioid chemicals such as morphine and heroin, and is involved in the brain's reward system.

"A pup needs opiate activity in the brain in order to find its mother rewarding," said Jaak Panksepp, a neuroscientist at Bowling Green University in Ohio who has studied the research. "Opiate activity is a very important player in social feelings."

When the receptor is absent, the mouse pups' brains cannot respond to the natural opioids that cement the bonds between infants and their mothers, the results suggest. Dr Moles said: "We hypothesise that the deficits in our knockout mice annihilate the natural association between reward and maternal stimuli, making these animals less sensitive to maternal separation."

The study, details of which are published today in the journal Science, may have important implications for social attachment disorders, particularly autism. Children with autism fail to form normal social bonds with their parents and the findings suggest that a problem in their opioid receptors may play a part in their condition.