A method for assessing a subjects risk for autism, the method comprising: (a) assaying plasma from the subject for the small molecule metabolites of a metabolomic signature, wherein the metabolomic signature comprises: (i) the twenty one metabolites homocitrulline, 2-hydroxyvaleric acid, cystine, aspartic acid, isoleucine, creatinine, serine, 4-hydroxyphenyllactic acid, citric acid, glutamic acid, lactic acid, dehydroepiandrosterone sulfate (DHEA sulfate or DHEAS), glutaric acid, 5-hydroxynorvaline, heptadecanoic acid, 5-aminovaleric acid lactam, succinic acid, myristic acid, 2-hydroxyvaleric acid, methylhexadecanoic acid, and 3-aminoisobutyric acid; or (ii) the twenty nine metabolites homocitrulline, glutaric acid, saccharopine, 5-aminovaleric acid, lactate, succinate, isocitrate, DHEAS, DHA, androsterone sulfate, 27-norcholesterol, Lyso PE, PE, long chain Fas, LysoPC, asparate, glutamate, acetylornithine, valine, isoleucine, ketoleucine, serine, homocysteic acid, valine, cystine, hydroxyacetone, phosphohydroxypyruvate, indole-3-lactate, and 3-amino isobutyrate; and (b) quantifying the amount of the small molecule metabolites of the metabolomic signature; wherein a statistically significant abundance difference as compared to non-autistic controls in the small molecule metabolites of the metabolomic signature indicates an increased risk of autism. (a) assaying plasma from the subject for the small molecule metabolites of a metabolomic signature, wherein the metabolomic signature comprises: (i) the twenty one metabolites homocitrulline, 2-hydroxyvaleric acid, cystine, aspartic acid, isoleucine, creatinine, serine, 4-hydroxyphenyllactic acid, citric acid, glutamic acid, lactic acid, dehydroepiandrosterone sulfate (DHEA sulfate or DHEAS), glutaric acid, 5-hydroxynorvaline, heptadecanoic acid, 5-aminovaleric acid lactam, succinic acid, myristic acid, 2-hydroxyvaleric acid, methylhexadecanoic acid, and 3-aminoisobutyric acid; or (ii) the twenty nine metabolites homocitrulline, glutaric acid, saccharopine, 5-aminovaleric acid, lactate, succinate, isocitrate, DHEAS, DHA, androsterone sulfate, 27-norcholesterol, Lyso PE, PE, long chain Fas, LysoPC, asparate, glutamate, acetylornithine, valine, isoleucine, ketoleucine, serine, homocysteic acid, valine, cystine, hydroxyacetone, phosphohydroxypyruvate, indole-3-lactate, and 3-amino isobutyrate; and (i) the twenty one metabolites homocitrulline, 2-hydroxyvaleric acid, cystine, aspartic acid, isoleucine, creatinine, serine, 4-hydroxyphenyllactic acid, citric acid, glutamic acid, lactic acid, dehydroepiandrosterone sulfate (DHEA sulfate or DHEAS), glutaric acid, 5-hydroxynorvaline, heptadecanoic acid, 5-aminovaleric acid lactam, succinic acid, myristic acid, 2-hydroxyvaleric acid, methylhexadecanoic acid, and 3-aminoisobutyric acid; or (ii) the twenty nine metabolites homocitrulline, glutaric acid, saccharopine, 5-aminovaleric acid, lactate, succinate, isocitrate, DHEAS, DHA, androsterone sulfate, 27-norcholesterol, Lyso PE, PE, long chain Fas, LysoPC, asparate, glutamate, acetylornithine, valine, isoleucine, ketoleucine, serine, homocysteic acid, valine, cystine, hydroxyacetone, phosphohydroxypyruvate, indole-3-lactate, and 3-amino isobutyrate; and (b) quantifying the amount of the small molecule metabolites of the metabolomic signature;