In a recent cabinet meeting, U.S. Health Secretary Robert F. Kennedy Jr. suggested a potential connection between male circumcision and autism, a claim that echoes a debate within certain corners of scientific and public discourse. While the assertion may seem surprising, a handful of scientific papers have explored this very question, examining statistical correlations and potential biological pathways. These studies, however, are not without significant limitations and have faced substantial criticism from other researchers in the field.
During the meeting, Kennedy addressed President Donald Trump with his concerns, linking the procedure to autism through the use of a common pain reliever. “There are two studies which show children who are circumcised early have double the rate of autism, it’s highly likely, because they were given Tylenol,” Kennedy told President Donald Trump in Thursday’s meeting. An examination of the scientific literature reveals the studies to which he was likely referring.
The first of these papers was an exploratory analysis published in the journal Environmental Health in 2013 by researchers Ann Z. Bauer and David Kriebel. The authors sought to investigate a hypothesis that the use of paracetamol, also known as acetaminophen and sold under brand names like Tylenol, during pregnancy or in early infancy might be a risk factor for autism spectrum disorder.
Their rationale was based on several observations. The prevalence of autism and the consumption of paracetamol both increased dramatically after 1980. Some studies also suggest children with autism may have a decreased capacity to metabolize the drug, potentially leading to oxidative stress and immune system effects.
Bauer and Kriebel noted that in the mid-1990s, medical guidelines began recommending the use of analgesics like paracetamol to manage pain for infant circumcisions, a procedure previously performed often without pain relief. This change in practice a provided unique opportunity. The researchers hypothesized that if paracetamol exposure increases autism risk, then populations with higher circumcision rates after 1995 might also show higher rates of autism in males. They proposed using circumcision rates as a proxy, or stand-in measure, for early life paracetamol exposure in boys.
To test their ideas, the researchers conducted two separate ecological analyses, which look for correlations in population-level data rather than in individuals. The first analysis compared prenatal paracetamol usage rates with autism prevalence rates in eight countries where data was available.
The second, more central analysis compared male circumcision rates against male autism prevalence rates. They divided their data into two time periods: one for boys born before 1995, when it was assumed paracetamol was rarely used for the procedure, and one for boys born after 1994, when pain management with paracetamol became more common.
The results showed positive correlations. In the first analysis, the researchers found a strong correlation (r=0.80) between a country’s average prenatal paracetamol use and its autism prevalence. In the second analysis of boys born after 1995, they found a very strong correlation (r = 0.98) between male circumcision rates and autism prevalence across nine countries. This trend suggested that a 10% increase in a country’s circumcision rate was associated with an increase in male autism prevalence of 2.01 per 1,000 boys.
A similar strong correlation (r=0.95) was found among 14 U.S. states. For boys born before 1995, the correlation was still present but weaker (r = 0.89), with a 10% change in circumcision rate associated with a much smaller increase in autism prevalence of 0.35 per 1,000 boys.
Prompted in part by Bauer and Kriebel’s findings, a different team of researchers conducted a follow-up study. Published in the Journal of the Royal Society of Medicine in 2015, the study by Morten Frisch and Jacob Simonsen aimed to test the hypothesis directly in a large national cohort. They noted that painful or stressful events in early life have been linked to long-term psychological consequences and that even with modern pain relief, circumcision involves procedural and postoperative pain. Their investigation sought to determine if this early life stressor could be an adverse outcome for some boys, specifically by increasing their risk of developing autism spectrum disorder.
Frisch and Simonsen conducted a national register-based cohort study in Denmark. They identified a group of 342,877 boys born between 1994 and 2003 and followed them for up to ten years. Using Denmark’s comprehensive national health registries, they identified which boys underwent ritual circumcision, which were diagnosed with autism spectrum disorder or other conditions like hyperkinetic disorder and asthma, and a host of other potential influencing factors such as birth weight, gestational age, and parents’ country of origin.
The analysis used statistical models to compare the risk of developing autism in circumcised boys versus intact boys, while attempting to control for cultural background and other perinatal risk factors.
The study found that, overall, ritually circumcised boys were 46% more likely to be diagnosed with autism spectrum disorder before age 10 compared to uncircumcised boys. The risk appeared highest for infantile autism diagnosed before age five, where circumcised boys had twice the risk of their intact peers.
The researchers also found an unexpected increased risk of hyperkinetic disorder among circumcised boys from non-Muslim families. By contrast, they found no association between circumcision and asthma, an outcome they checked for comparison.
The Danish study soon drew criticism. In a letter to the Journal of the Royal Society of Medicine published later in 2015, Brian J. Morris and Thomas E. Wiswell argued the claims were flawed. They pointed out that the statistical significance was marginal and the absolute number of affected boys was small.
They also highlighted a key issue with the data: the study recorded only 10.9% of boys from Muslim backgrounds as being circumcised, a figure they described as “improbable” and one that undermines the findings. Ritual circumcision is a near-universal religious and cultural practice for Muslim families. The actual circumcision rate in this demographic group should be close to 100%, not the 10.9% captured by the study’s methods.
The critics also questioned the study’s focus. “Since the Danish study was about pain why didn’t Frisch and Simonsen examine other painful conditions? Urinary tract infections are associated with excruciating pain, are common and very much higher in uncircumcised male infants and boys,” they wrote. They suggested that if the pain hypothesis were correct, autism should be associated with being uncircumcised, due to higher rates of these infections. They also proposed alternative explanations, such as the effects of anesthesia or the possibility that both early circumcision and autism diagnosis reflect higher levels of parental conscientiousness.
Seeking to bring clarity to a range of similar claims, a team of researchers led by Morris published a systematic review in the Journal of Evidence-Based Medicine in 2022. The aim of the review was to identify all the various psychological, psychophysiological, and psychosocial consequences attributed to circumcision in early childhood and to critically examine the quality of the evidence for and against these purported harms. The review was the first of its kind to systematically evaluate the breadth of these claims.
In their review, Morris and his colleagues strongly criticized the supposed link between circumcision and autism. They reiterated that the statistical association in the Danish study was weak and that the very low reported circumcision rate among Muslim boys cast doubt on the data’s validity.
The reviewers emphasized the point made by the original study’s author, Ann Bauer, that her 2013 paper was about paracetamol, not circumcision itself. The association with autism was much stronger in the time period after paracetamol use became common for the procedure, which they argue supports the paracetamol hypothesis over a pain-trauma hypothesis.
Other potential explanations for the findings in the Danish study include confounding factors, such as cultural differences and the fact that more attentive parents might be more likely to both have their sons circumcised and seek an early autism diagnosis.
Overall, Morris and his colleagues concluded in their review that “the highest quality evidence suggest that neonatal and later circumcision has limited or no short-term or long-term adverse psychological effects.”
All the studies involved acknowledge significant limitations, a point that is central to understanding the current state of the science on this topic. A major issue is the lack of research that can establish a direct causal link. Bauer and Kriebel were clear that their work was a “hypothesis generating exploratory analysis.” As an ecological study, it looked for patterns in large population groups, not in individuals. This type of analysis can never prove that one thing causes another. It can only show that two trends appear to move in tandem within a population.
The core weakness of an ecological study is what is known as the “ecological fallacy.” This is the error of assuming that relationships observed for groups necessarily hold true for individuals. For example, a city with a high number of libraries might also have a high rate of college graduates, but this does not mean that every person who uses a library is a college graduate.
In the context of the 2013 study, a high rate of circumcision in a country and a high rate of autism do not prove that circumcision causes autism in any given child. There could be countless other hidden factors, or confounders—such as differences in healthcare systems, access to diagnostic services, parental reporting habits, or genetic predispositions in certain populations—that could be the real reason for the statistical correlation. The use of circumcision as a proxy for paracetamol use adds another layer of indirectness, further weakening any causal interpretation.
The second major limitation across this body of research is the scarcity of high-quality longitudinal evidence. A longitudinal study is considered a much more powerful tool for investigating potential causes because it follows the same group of individuals forward in time. An ideal study on this topic would enroll a large cohort of newborn boys, carefully document which ones are circumcised (including details on pain management used), and then follow all of them for many years to see who develops Autism Spectrum Disorder. This design allows researchers to establish a clear timeline—that the proposed cause happened before the effect—and to systematically collect data on many other potential contributing factors over the course of a child’s development.
The research that exists is largely not of this type. The Danish study by Frisch and Simonsen was a step closer, as it was a cohort study that looked back at registry data. However, as the authors and critics noted, it had its own problems with incomplete data, potentially misclassifying boys who were circumcised outside the public health system.
The 2022 systematic review by Morris and colleagues underscored this problem, concluding that the field is hampered by studies that are mostly cross-sectional. A cross-sectional study is a snapshot of data — it captures data from a group of people at a single point in time. It can show an association but it cannot untangle the sequence of events or rule out the possibility that other life experiences are the true cause. Without robust longitudinal studies, it is difficult to separate a potential effect of a single procedure in infancy from the complex interplay of genetics and a lifetime of other environmental influences.
In a statement posted to the social media platform X (formerly Twitter) on Friday, Kennedy said that one of the studies he was referencing was a 2025 preprint research article. A preprint is a version of a scientific manuscript that has not yet been subjected to peer review, the formal evaluation process where independent experts in the same field scrutinize a study’s methods, data, and conclusions before it is published in a scientific journal. This particular preprint was authored by a team of researchers including M. Vishnu Patel and William Parker.
“As usual, the mainstream media attacks me for something I didn’t say in order to distract from the truth of what I did say,” Kennedy wrote. “An August 2025 http://Preprints.org review by Patel et al. directly validates my point that the observed autism correlation in circumcised boys is best explained by acetaminophen exposure, not circumcision itself.”
“So once again, MSM chooses to character assassinate me instead of educating Americans by digging into the science. The paper I was referring to identifies the 2015 Danish circumcision study as ‘the most compelling ‘standalone’ evidence that acetaminophen triggers autism in susceptible babies and children.'”
The preprint does not present a new experiment but is instead a systematic analysis of previously published scientific literature. The central argument of the paper is that “overwhelming evidence shows that exposure of susceptible babies and children to acetaminophen (paracetamol) triggers many if not most cases of autism spectrum disorder.” The authors state that their goal was to understand why this conclusion has not been widely accepted by the medical and regulatory communities. They assert that the failure to accept this link is due to mishandling of evidence within the scientific literature itself.
The preprint authors identify three primary ways they believe the evidence has been mishandled by other researchers. The first error, they state, is a fundamental statistical mistake. They argue that many studies incorrectly treat factors like infection, maternal stress, or genetics as separate “confounding” factors. The preprint’s authors contend these are actually “interacting” variables that create a state of oxidative stress, making a child susceptible to acetaminophen. By statistically “adjusting” for these factors, they claim, researchers have inadvertently erased the very signal they should be looking for—the combined effect of acetaminophen and susceptibility.
A second problem identified is that researchers often look at too narrow a slice of the evidence. For example, a study might only examine acetaminophen use during pregnancy while ignoring the post-birth period, which the preprint authors argue is a time of high sensitivity. They also note that many papers discussing the topic fail to cite what they consider to be seminal earlier studies, including the papers by Frisch and Simonsen and by Bauer and Kriebel. Finally, the paper claims that key earlier studies suggesting a link were subjected to “irrational criticisms” that used invalid reasoning to unfairly dismiss important findings, preventing the scientific community from taking the hypothesis seriously.
