US researchers analysed how infants looked at faces from birth to the age of three.
They found children later diagnosed with autism had shown diminished eye contact - a hallmark of autism - in the first few months of life.
The findings, reported in Nature, raised hope for early interventions to tackle autism, said a UK expert.
In the study, researchers led by Emory University School of Medicine in Atlanta used eye-tracking technology to measure the way babies looked at and responded to social clues.
They found infants later diagnosed with autism had shown a steady decline in attention to the eyes of other people from the age of two months onwards, when watching videos of natural human interactions.