Concerns about a rising prevalence of autism spectrum disorders (ASD) began to emerge in the late 1990s and early 2000s. An article published in the Journal of Environmental Medicine (2000; Volume 10:261–266) summarized early surveillance data suggesting higher reported rates of autism compared with earlier decades. Researchers cited estimates from the Centers for Disease Control and Prevention, including reports of approximately 40 cases per 10,000 children in Brick Township, New Jersey, and higher prevalence estimates from parts of the United Kingdom, including Cambridgeshire and London.
The article noted that between 1987 and 1998, reported autism prevalence increased substantially, depending on diagnostic definitions used. Earlier studies published between 1966 and 1988 reported average prevalence rates of approximately 4.3 per 10,000, while studies published between 1989 and 1998 reported higher estimates, averaging 7.2 per 10,000. These findings raised questions about whether the increase reflected a true rise in incidence or changes in diagnostic criteria, awareness, and reporting practices.
At the time, several hypotheses were proposed to explain the increase in reported autism diagnoses. Some researchers and advocacy groups suggested that expanded diagnostic criteria, improved recognition, and increased access to services played a major role. Others raised concerns about environmental exposures, including heavy metals such as mercury, and speculated about a possible relationship with childhood immunizations. These concerns were amplified by congressional hearings and public testimony in the late 1990s and early 2000s.
In response, numerous epidemiologic and biologic studies were conducted to evaluate these hypotheses. Large population-based studies published in major medical journals examined autism rates in relation to vaccination status, thimerosal exposure, and mercury biomarkers. These studies consistently found no evidence that vaccines or vaccine-derived mercury cause autism. Research comparing populations before and after thimerosal removal from vaccines, as well as comparisons across countries with differing vaccine formulations, did not demonstrate corresponding changes in autism prevalence.
Current scientific consensus holds that the increase in reported autism prevalence is largely explained by changes in diagnostic criteria, heightened awareness, improved screening, and broader inclusion within the autism spectrum, rather than a new environmental cause. Autism is now understood to be a neurodevelopmental condition with strong genetic contributions, influenced by complex interactions between genes and early developmental factors.
