Autism severity increases between first and second born affected siblings, especially when births occur within two years of each other, according to a study published by Loren Martin, Ph.D., professor and director of research for the Department of Graduate Psychology at Azusa Pacific University. APU graduate psychology student Narges Horriat assisted Martin with the study.The paper appears in the November 30, 2012, issue of PLOS ONE, a leading peer-reviewed scientific journal. View the article online.

Autism Study Highlights:

  • For the first time, this study finds a link between birth order and the severity of autism symptoms in affected siblings. Overall, symptoms are more severe in the second child born with autism within a family than the first born.
  • The study also confirms previous reports indicating IQ decreases between first and second born children with autism.
  • The study results indicate that female siblings were more severely impacted by autism symptoms than males.
  • This is the first study to measure birth order and birth interval effects on autism using the Social Responsiveness Scale (SRS), a validated measure of autism severity.
  • The difference in autism severity across birth order as measured with the SRS is only observed when the birth interval is less than two years.
  • The data suggests a causal mechanism of autism acts in a dose-response relationship with a heightened effect across pregnancies but which dissipates with increased time between pregnancies.
  • This study represents the largest analysis of birth order and birth interval effects on Autism Spectrum Disorder (ASD) published to date.
  • The research involved more than 300 sibling pairs diagnosed with an ASD from the Autism Genetic Resource Exchange database. Information was collected over the past 10 years. The mean age was 11 for first affected children and eight for second affected children.

“Autism is diagnosed over 10 times more frequently today than it was in the early 1990s, with an incidence occurring now in around one percent of the population,” said Martin. “While this rise is at least partially related to an increase in awareness and broadened diagnostic criteria, the role of environmental factors cannot be ruled out. The study of families with multiple affected children can provide clues about the causes of ASD. The findings from this study 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.”

Martin, who began teaching at APU in 2006, holds a Ph.D. from the University of Tennessee Health Science Center where he studied the role of cerebellar neuropathology in autism. Martin completed postdoctoral work at the MIND Institute (Medical Investigation of Neurodevelopmental Disorders) at the University of California, Davis.

His research was featured in the January 22, 2013 Simons Foundation Autism Research Initiative (SFARI) newsletter.