A recent study conducted by Loren Martin, Ph.D., professor and director of research for the Department of Graduate Psychology at Azusa Pacific University, links birth order to the severity of autism. Drawing from more than 300 sibling pairs diagnosed with Autism Spectrum Disorder from the Autism Genetic Resource Exchange database, the study presents the largest analysis of birth order and interval effects on autism. Using the Social Responsiveness Scale (SRS) to compare symptoms and severity between siblings, Martin found both birth order and interval to be associated with autism. Children born within two years of siblings with autism were found to have more severe symptoms than their older siblings. The study findings show that IQ is similarly associated with birth order, with the second born population showing generally lower IQ than first born among the autistic sibling pairs. Martin explains the significance of his findings saying, “The findings suggest a dosage-type effect in some cases of ASD in which genetic and/or environmental factors accumulate across pregnancies, leading to a more severe manifestation of ASD symptoms.”[i] The study was published in the November 30, 2012 issue of the scientific journal PLOS ONE.
[i] “APU Articles.” Psychology Professor Provides Clues about Autism Causality through Birth Order Study. N.p., n.d. Web. 14 May 2013. <http://www.apu.edu/articles/20247/>.