Because autism is rarely diagnosed before two years of age, little is known about its early symptoms and causes. In order to determine the earliest manifestations of the condition, recent interest has focused on infants at genetic risk for autism. Current evidence indicates that overt behavioural symptoms emerge around the end of the first year. However, studies using laboratory brain function measures have reported differences in groups of infants at-risk compared with low-risk controls during their first year. Some infants displaying such early differences, however, do not subsequently receive a diagnosis. As the search for early markers continues, infants at-risk present a persuasive model for gene by environment interactions leading to variable developmental pathways.