We provide a systematic review of epidemiological surveys of autistic disorder and pervasive developmental disorders (PDDs) worldwide. A secondary aim was to consider the possible impact of geographic, cultural/ethnic, and socioeconomic factors on prevalence estimates and on clinical presentation of PDD. Based on the evidence reviewed, the median of prevalence estimates of autism spectrum disorders was 62/10 000. While existing estimates are variable, the evidence reviewed does not support differences in PDD prevalence by geographic region nor of a strong impact of ethnic/cultural or socioeconomic factors. However, power to detect such effects is seriously limited in existing data sets, particularly in low‐income countries. While it is clear that prevalence estimates have increased over time and these vary in different neighboring and distant regions, these findings most likely represent broadening of the diagnostic concets, diagnostic switching from other developmental disabilities to PDD, service availability, and awareness of autistic spectrum disorders in both the lay and professional public. The lack of evidence from the majority of the world's population suggests a critical need for further research and capacity building in low‐ and middle‐income countries. Autism Res 2012, 5: 160–179. © 2012 International Society for Autism Research, Wiley Periodicals, Inc.

There are no epidemiological data on autism for Mexico. This study was conducted to generate a first estimate of ASD prevalence in Mexico. We surveyed children age eight in Leon (Guanajuato). The sample was stratified in two strata: (1) children having special education and medical records (SEMR; N = 432) and (2) children attending regular schools (GSS; N = 11,684). GSS children were screened with the SRS and those with the highest scores were invited to a diagnostic evaluation. The final sample comprised 36 children (80.6 % male) who had confirmed ASD. A third had intellectual disability, 25 % were non-verbal, 69 % had co-occurring behavioral problems. The prevalence overall was 0.87 % (95 % CI 0.62, 1.1 %). This survey provides an estimate for ASD prevalence in Mexico that is consistent with recent studies.

In order to conduct the screening phase of the first epidemiological survey of autism spectrum disorders (ASDs) in Mexico, we needed a screening tool to detect autistic symptomatology in a large sample of school-age children. We used the Spanish version of the Social Responsiveness Scale (SRS). We recruited a clinical sample of 200 children (81% males; mean age: 7.4 years) with a confirmed diagnosis of ASDs and a sample of 363 control children (59.5% males; mean age: 8.5 years) without ASDs. Three-way analyses of variance (ANOVAs) identified a main effect of clinical status (ASDs vs. controls) for both parent and teacher scales, but no gender or age effect. The mean total and subscale raw scores were significantly different between the clinical and control groups for the parent and for the teacher SRS (P < 0.001). The internal consistency of the SRS was excellent. Receiver operating characteristic (ROC) analyses showed excellent discriminant validity of the SRS in the Mexican sample (area under the curve: 0.962 for the parent, 0.960 for the teacher). ROC curves were also used to determine which cutoff would provide the best trade-off between sensitivity and specificity. Mexican SRS scores were significantly higher than in the U.S. and German population for typically developing children but comparable for clinically referred subjects. The SRS is an acceptable screening instrument for epidemiological studies of ASDs in Mexico. Its psychometric properties are excellent and comparable to those derived from North American and other samples.