Babies with autism look at their mother's eyes and mouth much less than their peers - a discovery that scientists say could lead to new treatments for the condition.

The way in which infants see the world even before they can reach, crawl or walk is in the genes - and impaired in those with the neurological disorder.

Now a study has shown for the first time what youngsters choose to look at and what they don't is heritable.

And the way they visually engage with others in social situations is altered in autism.

Reduced attention to other people's eyes and faces is a behaviour associated with the condition and is often used to help diagnose it.

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A series of eye-tracking experiments in 338 toddlers assessed variation in viewing social scenes - including levels of attention to faces and face-like stimuli - and the timing, direction and targeting of individual eye movements.

Participants included 166 identical and non-identical twins, 88 children diagnosed with autism and 84 others that acted as controls.

Each child watched videos that showed either an actress playing the role of a caregiver and speaking directly to the viewer or scenes of children playing.

They could look at the on-screen characters' eyes, mouth, body or surrounding objects.

Special software captured how often the children looked at different regions - as well as the timing and direction of eye movements.

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The study published in nature showed identical twins were also nearly identical in the way they watched the videos.

How much one identical twin looked at another person's eyes was almost perfectly matched by his or her co-twin. That match fell to only about 10 percent, for non-identical twins.

Identical twins were also much more likely to move their eyes at the same moments in time, in the same directions, towards the same locations and the same content - mirroring one another's behaviour to within as little as 17 milliseconds.

Dr Warren Jones, an autism expert at Emory University in the United States, said: "These data show us that a child's genes shape the way she sees the world.

"And how a child looks at the world is how it learns about the world. Each eye movement - happening every half-second--shapes brain development.

"So you can imagine these effects rippling forward, creating the way a child sees and understands her world.

"This changes our understanding of how children experience their environment and changes our understanding of what forces shape that experience.

"This shows us our genetic biology exerts a strong and pervasive influence on what we would otherwise imagine to be unique individual perceptions."

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The effects persisted as the children grew. When the twins were tested again, more than a year later, the same effects were found.

Identical twins remained almost perfectly matched in their levels of looking, but non-identical twins became even slightly more different than before.

Professor John Constantino, a psychiatrist at Washington University in St Louis, said: "Not only is this aspect of social visual engagement under stringent genetic control but the trait is very stable.

"Taken as a whole this means we have a new way to trace direct effects of genetic factors on early social development.

"This is a mechanism by which genes actually modify a child's life experience.

"And, because of that, this creates a new opportunity to design interventions to ensure that children at risk for autism acquire the kind of social environmental inputs that they need."

Levels of eye- and mouth-looking - the same traits that showed the greatest degree of genetic influence in typically-developing children - were substantially reduced in children with autism.

In autism, the levels were low enough, and different enough, that they could be used to mathematically identify the vast majority of children who did or did not have autism.

Dr Jones said: "This is an extremely important step.

"It's a link between the behavioural presentation of autism and an objective, quantifiable trait, one that emerges very early in infancy, and one that we now know - with these results - is directly influenced by genetics.

"How a child looks at the surrounding world, with or without autism, is directly traceable to its genetics."

Autism is known to be primarily caused by genetic factors, and earlier research by members of the team showed that babies who look progressively less at people's eyes, beginning as early as 2-6 months of age, are more likely to have autism.

Now, with these new results in twins, the team has found a specific behaviour that is highly influenced by genetics and directly linked to autism risk.

Dr Lisa Gilotty, chief of the Research Program on Autism Spectrum Disorders at the National Institute of Mental Health - one of the agencies that supported the research, said: "Studies like this one break new ground in our understanding of autism spectrum disorder.

"Establishing a direct connection between the behavioral symptoms of autism and underlying genetic factors is a critical step on the path to new treatments."

Autistic spectrum disorders cause problems with communication and social sskills and affect more than 580,000 people in the UK.