Study finds surprising health differences between first- and second-born children

The order in which children are born may have implications that go beyond family dynamics and personality, with new research suggesting that birth order is associated with differences in the risk of hundreds of diseases and health conditions.

A study published in the Nature Health journal by researchers from the University of Chicago and Columbia University examined whether the number of children in a family and their position among siblings could be linked to differences in health outcomes, as the Science Alert outlet notes.

The researchers found that first-born children were more likely to have allergies and certain neuropsychiatric conditions, including autism, attention deficit hyperactivity disorder (ADHD), Tourette’s syndrome and obsessive-compulsive disorder (OCD).

Second-born children, meanwhile, showed higher statistical associations with conditions including gastrointestinal problems, migraines, shingles and substance-use disorders.

"The birth order, defined as the ordinal position of a child among siblings, has fascinated researchers for more than a century," the researchers wrote in their 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."

The study analyzed US health insurance records covering more than 10.3 million children who had one sibling. The researchers used the data to examine correlations between being a first-born or second-born child and 569 different diseases included in a so-called "disease atlas."

Of the 569 conditions examined, 150 showed different associations between first-born and second-born children, including the conditions highlighted by the researchers.

The researchers compared siblings within the same family as well as first- and second-born children from different families. This approach was intended to reduce the influence of other factors that could affect health outcomes, such as household income or the presence of pets.

"The disease atlas view reveals that birth-order effects are not confined to a few well-studied conditions but instead pervade nearly every clinical domain tested, with a striking concentration of first-born excess in neuropsychiatric conditions and second-born excess in digestive and musculoskeletal diseases," the researchers wrote.

Age gap also appears to matter

The researchers found that the age difference between siblings was associated with some of the observed patterns. A larger age gap, for example, was linked to a higher likelihood of autism among first-born children, while appearing to reduce the likelihood of first-borns developing allergies that their younger siblings did not have.

The findings do not establish that birth order itself causes particular diseases. Researchers said further work is needed to determine why these associations exist and to separate birth-order effects from other variables, including parental age, which naturally differs between siblings.

The study adds to a long-running body of research into the potential effects of birth order. Previous studies have suggested that birth order may influence characteristics such as IQ, while finding little evidence that it determines career choices.

The new findings suggest that the relationship may extend beyond commonly discussed differences between older and younger siblings, including the possibility that younger children develop different immune responses after being exposed to microbes brought home by their older siblings.

"The convergence of evidence from between-family and within-family designs, combined with validation controls and robustness analyses, is consistent with a mixture of physiological, immunological and social mechanisms operating throughout the human disease phenome."

By Nazrin Sadigova