Understanding the natural history of a disease is an important framework to have. It not only is critical for prognosis, but also informs us about diagnostic and screening strategies, is important to assessing interventions, and provides clues to causation.

There has been much debate about the early course of autism, specifically the earliest age at which autism may be detected. At present scientific evidence suggests that autism is dominantly genetic, and so researchers expect that there may be early signs of autism even in infancy. Traditionally, however, autism is not diagnosed until age 2-3, when parents bring their children to medical attention, or when signs are detected on routine well-child visits or in day-care.

Retrospective studies, largely involving review of home movies, have suggested that autism can be diagnosed as early as 6-12 months, suggesting that parental report is not an adequate screen because subtle signs are hard to detect without rigorous observation.

Now a group has published the first prospective study to address this question. They followed 25 children who were later diagnosed with autism spectrum disorder (ASD) (22 of which were high risk) and 25 low risk children who were later determined to have typical development (TD). They found:

These results suggest that behavioral signs of autism are not present at birth, as once suggested by Kanner, but emerge over time through a process of diminishment of key social communication behaviors. More children may present with a regressive course than previously thought, but parent report methods do not capture this phenomenon well. Implications for onset classification systems and clinical screening are also discussed.

More precisely, they carefully assessed the children, counting instances of eye contact and smiling, for example, and found that there were no statistically significant differences between the groups at 6 months, but that almost all measures were reduced in the ASD group by 12 months.

The study is rigorously designed, and its primary weakness is that it is a bit small. The authors also acknowledge that future studies should utilize longer periods of recorded observation. I would add that more frequent assessments between 6 and 12 months would help specify when the earliest divergence takes place. Replication of the study is therefore desirable.

But what these results indicate is that clear signs of autism emerge between 6 and 12 months of age. Further, social skills tend to be regressive in ASD between 6 and 18 months of age. It was previously thought that social regression was less common in ASD, but this study suggests it is the rule, not the exception. Meanwhile, language skills did not regress in this study, they continued to improve in the ASD group, just on a slower curve than the TD group.

Further the study documented that the parents generally did not recognize the social regression between 6 and 18 months. The authors refer to the well-known phenomenon of telescoping in which patients or family members will recall the onset of symptoms as being much more recent than they actually were.

Prior studies using home movies have shown that signs of autism can be detected between 8-12 months. A study looking at head circumference found statistical differences prior to 12 months. And one study looking at movements found differences between 4-6 months. So it seems the consensus of current evidence is that objective and detectable signs of autism emerge between 6-12 months. This study does not support detection prior to 6 months, but other studies do suggest this might be possible.

This study has implications for diagnostic categories of autism (specifically distinguishing regressive forms of autism from non-regressive forms, since most of the ASD children in this study showed social regression). It also is very informative regarding screening strategies. It suggests no utility to screening children prior to 6 months of age. Further, since ASD children continued to separate from TD children through 3 years of age, screening for ASD should not stop at 2 but continue to at least 3.

The authors, however, do not discuss one very significant implication of this study (although an implication already raised by prior studies demonstrating early signs of ASD) – the observation made by many parents that ASD symptom onset correlates with certain vaccinations. Many children are diagnosed between the age of 2 and 3, during the height of the childhood vaccine schedule. This lends itself to the assumption of correlation and causation on the part of some parents. The phenomenon of telescoping, whereby memories of time contract, will tend to reinforce this false correlation.

What this and other studies show is that not only is the assumption of causation fallacious, the observation of correlation is likely flawed as well. The true onset of autism in most ASD children likely began a year or two prior to the vaccines that are blamed as the cause.

This point was made most dramatically by the Cedillo case – one of the test cases brought before the Autism Omnibus court alleging vaccine injury causing autism. Cedillo’s parents alleged that their child developed autism as a result of a combination of the MMR vaccine and thimerosal from other vaccines. In courtroom testimony, however, experts were able to show with home movies that Cedillo showed clear signs of autism as an infant, prior to ever receiving the MMR vaccine.

The current study adds nicely to the growing consensus that the true clinical onset of ASD is between 6 and 12 months of age. Whether or not there are biological markers of ASD prior to that remains to be seen, but is not unlikely. Early and fairly uniform onset is consistent with genetic causes of ASD, rather than environmental causes.


As several commenters have pointed out, I erred when I wrote that “Many children are diagnosed between the age of 2 and 3, during the height of the childhood vaccine schedule.” What I meant to convey was that this time period is still within the childhood vaccine schedule, and therefore it is likely that when parents first notice the signs of autism their child would have been recently vaccinated by chance alone.  The point remains, however, that many parents blame their children’s autism on vaccines they received after the true onset of symptoms.

Here I was thinking primarily of the MMR vaccine, the first to be blamed for autism (thanks to Andrew Wakefield). As you can see from the vaccine schedule, the first MMR vaccine in the series is scheduled for 12 months – after most children already have detectable signs of autism. This is why I used the Cedillo case to reflect this point.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.