Two professors at Yale University believe they have identified the purpose. R. Polimanti and J. Gelernter of the departments of psychiatry and genetics looked at genetic data on five psychiatric conditions: autism (ASD), attention deficit hyperactivity disorder, major depressive disorder, schizophrenia, and bipolar disorder. Other researchers had already identified gene variants that seem to connect with each of these conditions, and the Yale scientists analyzed the data to determine whether the relevant genes were dying out or becoming more stable, or "fixed." Only autism and schizophrenia showed a high likelihood of being associated with advantageous (in an evolutionary sense) genes, and only autism “survived” all of the statistical tests they ran. Polimanti and Gelernter hypothesize that “certain ASD risk alleles [gene variants] were under positive selection during human evolution due to their involvement in neurogenesis and cognitive ability.” In other words, some of the gene variants associated with autism are also significantly associated with high intelligence. “Smart” genes are advantageous from an evolutionary standpoint, so they persist.
