The invention relates to the determination of phosphorylation levels of some specific salivary peptides (statherin, histatin 1, acidic proline-rich proteins, both in whole form (PRP-1 type) and in truncated form (PRP-3 type) as a non-invasive diagnostic marker with a >40% sensitivity for defining autism spectrum disorders (ASDs) patients, methods for performing said determination, a kit for the non-invasive diagnosis of at least 40% of autism cases, containing means for such a determination, and the use of such a determination for therapeutical research on autism.