Whenever a new sibling arrives in a family, the dynamic changes in all sorts of ways.There's an extra seat at the dining table, less room on the couch, and another opinion to consider at family meetings (at least when they reach a certain age).In a study published in Nature Health, researchers from the University of Chicago and Columbia University have explained how the number of children in a family, and the order they're born in, appears to be linked to potential impacts in terms of health and disease, too.First-born children are more likely to have allergies, and conditions including autism, attention deficit hyperactivity disorder ( ADHD), Tourette's, and obsessive-compulsive disorder (OCD), the study found.150 diseases were statistically more likely for first-borns or second-borns. (Kramer et al., Nat. Health, 2026)Meanwhile, kids who are born second are statistically more likely to have stomach problems, migraines, shingles, and issues with substance abuse. "The birth order, defined as the ordinal position of a child among siblings, has fascinated researchers for more than a century," write the researchers in their published paper."Theorists in the early years proposed that first-borns receive greater parental investment and face higher expectations, while later-borns develop in the immunological and social wake of their older siblings."These ideas have generated a rich but fragmented empirical literature, with individual studies examining birth order in relation to specific diseases or developmental outcomes."