Firstborns More Likely Diagnosed with Autism and Allergies, New Study Reveals
A groundbreaking study analyzing health data from over 10 million siblings across 5 million families has confirmed that firstborn children are more prone to neurodevelopmental issues including autism, ADHD, and allergies compared to their younger siblings. The results were published this week on medRxiv, offering urgent insights into how birth order affects health risks.
Lead author Benjamin Kramer and his team conducted the largest examination yet of 569 medical conditions using administrative insurance claims data. Their findings show a clear pattern: firstborns have an increased likelihood of being diagnosed with childhood psychoses and acne, alongside autism and allergies, while secondborns face higher diagnoses of substance use disorders, shingles, and gastrointestinal issues.
Sibling Age Gap Influences Disease Risks
A surprising discovery involved the age gap between siblings. When the age difference was less than four years, children experienced lower rates of allergies and asthma. The study’s co-author noted this likely occurs because siblings close in age share more germs early on, boosting their immune defenses, in line with the so-called hygiene hypothesis.
“Firstborns’ immune systems may be less exposed to germs early in life, especially if they don’t attend daycare, which can cause higher allergy and asthma risks due to overreaction to allergens,” said the co-author.
Data Analysis Weighs Socioeconomic and Reporting Factors
Experts who reviewed the research praised its scale and design, particularly how it controlled for socioeconomic and genetic factors by comparing siblings within and across families. However, they also cautioned that insurance claims data might reflect parental behavior—parents may be more proactive seeking diagnoses for their firstborn, while symptoms in subsequent children could be overlooked.
University of Houston associate professor Damian (last name withheld) noted, “You can’t get a diagnosis if you don’t seek it,” emphasizing that firstborn children’s higher diagnosis rates could partly reflect increased medical attention.
The study also excluded families without a second child to avoid bias from “reproductive stoppage” where parents may halt childbearing after a severe diagnosis. Even after this adjustment, autism rates remained higher among firstborns.
Why Secondborns Show More Substance Use Disorders
Regarding the elevated substance use disorder diagnoses in secondborns, researchers suggest early exposure to older siblings’ behavior rather than innate risk-taking traits might explain the trend. Damian disputed claims linking birth order and risk-taking, saying existing evidence shows no straightforward connection.
Small Differences, Big Population Impact
While these birth order differences are statistically small on individual levels, population-wide they may significantly influence public health trends. The study’s thorough approach and extensive dataset add weight to these findings and call for greater awareness among parents and healthcare providers.
What This Means for Families and Healthcare
These new findings highlight the need for parents, pediatricians, and policymakers to consider birth order and sibling spacing in health risk assessments. Understanding how birth order affects risks for conditions like autism and allergies could guide earlier interventions and tailored healthcare strategies.
With almost half of American families having multiple children, this research could influence how doctors approach screenings and treatments, potentially improving early detection and support for at-risk children naturally linked to their position in the family.
“The researchers were methodologically thorough. They’re not just trying to tell a nice story but to really get at the right answer,” said Rohrer, an independent expert reviewing the study.
This research emerges amid growing efforts to map genetic, environmental, and familial factors that shape childhood development and disease. It also underscores the value of big data in uncovering subtle yet meaningful health trends.
As families nationwide watch for signs of neurodevelopmental disorders or allergies in young children, these findings offer a new lens to understand potential vulnerabilities—especially for firstborns.
