Younger siblings are often portrayed as reckless, scatterbrained and less put-together than their older siblings. Social science researchers have underpinned these portrayals with data, showing that younger siblings obtain, on average, slightly less education and earn slightly less in adulthood.

The explanation has often been that firstborns get more attention and quality time from parents than their younger siblings, resulting in the latter acting out. But a new study from Northwestern University suggests there may be a more immediate reason: germs.

Using Danish administrative data covering all firstborn and second-born siblings born in Denmark between 1981 and 2017, researchers found that younger siblings are two to three times more likely than older siblings to be hospitalized for a respiratory illness in the first months of life—largely because older siblings bring home viruses such as respiratory syncytial virus (RSV).