Birth Order Linked to Autism Risk in Eldest Children

Are you the eldest child? If so, recent data suggests you face a higher chance of being autistic, while your younger siblings might struggle more with addiction, according to a new study. For decades, experts have argued over whether birth order shapes who we become as people. Now, research involving over 10 million Americans implies that where you stand in the family could also dictate your long-term health outcomes. Scientists uncovered connections between family position and risks for 150 different conditions, ranging from autism to digestive issues. Firstborn kids were indeed more prone to neurodevelopmental problems like ADHD, Tourette's syndrome, and obsessive-compulsive disorder. They also showed elevated rates of food allergies, hay fever, asthma, and acne compared to their younger brothers or sisters.

Second-born children appeared less likely to receive diagnoses for many of these neurological conditions. The team found they were 43 percent less likely than firstborns to develop specific autism-related disorders. Yet this group faced greater risks in other areas, including digestive troubles, musculoskeletal pain, infectious diseases, substance abuse issues, migraines, stomach problems, kidney infections, and herpes zoster, the virus responsible for shingles. The study highlighted that firstborns had the clearest links to brain development and allergy-related ailments, while second-borns carried higher rates of those gastrointestinal and immune challenges. Substance abuse stood out as one of the strongest associations identified across the board.

These results, which appeared in Nature Health, drew from medical records spanning more than 10 million individuals across 5.1 million US families. Researchers noted it is unlikely any single factor explains these patterns so neatly. Instead, a mix of environmental exposures, parental attention styles, and genetic interactions likely drives the differences seen between siblings born at different times.

Experts argue that distinct conditions likely stem from unique biological and environmental factors shifting with every child born into a family. For instance, younger siblings face more viruses and bacteria thanks to their older brothers and sisters. This exposure fits the hygiene hypothesis, which might explain why these kids show fewer allergy symptoms compared to first-borns. In contrast, neurodevelopmental issues like autism and ADHD could trace back to subtle shifts in the prenatal environment between pregnancies, researchers suggest. Family dynamics also matter. Parents often treat first- and later-born children differently. Younger siblings grow up in a social world shaped by older brothers and sisters plus changing expectations at home.

Professor Andrey Rzhetsky from the University of Chicago told Newsweek that no one should alter a clinical decision just because a child is the first to arrive. A chart published in Nature Health illustrates which conditions first-born kids are more likely to develop compared to second-borns. The study linked 150 conditions, including autism and ADHD, to birth order, suggesting where someone falls in the family lineup might influence their health risks. Rzhetsky warned against seeing these findings as a direct prediction of an individual child's future health. He noted that birth order is interesting because it is a universal exposure across the entire population, not a tool to forecast any single kid's medical fate.

These results arrive just months after another study indicated younger siblings really do have it easier. Scientists at Australia's Monash University found parents tend to become more lenient with later-born kids, relaxing rules on everything from screen time to homework. Consequently, younger siblings spent more time on digital devices and less time on enrichment activities like reading, music, and homework than their first-born counterparts. The researchers said these findings suggest family dynamics change as families grow. This shift could help explain why birth order has long been connected to behavioral differences and perhaps even health outcomes.