ASD averages a 4.3:1 male-to-female ratio in diagnosis. The number of children known to have autism has increased dramatically since the 1980s, at least partly due to changes in diagnostic practice; it is unclear whether prevalence has actually increased; and as-yet-unidentified environmental risk factors cannot be ruled out. The Centers for Disease Control’s Autism and Developmental Disabilities Monitoring (ADDM) Network reports that in 2014, approximately 1 in 59 children in the United States (1 in 37 boys, and 1 in 151 girls), has been identified with an autism spectrum disorder (ASD). This estimate is a 15% increase from the 1 in 68 rate in 2010, 86% increase from the 1 in 110 rate in 2006 and 154% increase from the 1 in 150 rate in 2000. Diagnostic criteria of ASD has changed significantly since the 1980s; for example, U.S. special-education autism classification was introduced in 1994.