Sibling viralry
Why eldest siblings are brainier
A new study finds that sickness may play a role
SORRY, YOUNGER siblings.
On standard measures of success, such as educational achievement and income, first-borns do better.
Why? Stereotypes cast eldest children as responsible and younger ones as rebellious—but large-scale studies find no meaningful link between birth order and personality types.
New research points to something rather different: germs.
The gap in success between older and younger siblings has been pored over for decades.
In 2005 a study of Norway’s population found that younger siblings tend to drop out of education earlier than first-borns, regardless of family size and gender.
They also earn less as adults, with the gap widening for each subsequent child.
Younger sisters are more likely to become pregnant in their teens.
A new paper, by researchers in America, China and Denmark, provides an explanation.
Children often get sick, especially when very young.
The authors wondered whether older children could act as vectors, exposing their parents and vulnerable infant siblings to illness.
Using Danish administrative data, the researchers concluded that younger siblings were two to three times more likely than older siblings to be hospitalised for severe respiratory illnesses in their first year.
The effects of early health shocks seem to linger.
Sickness can hinder brain development directly (by causing inflammation) and indirectly (by diverting energy from the organ, to fight disease).
The authors found a causal link between early exposure to illness and lower wages in adulthood.
Other studies have found that fevers and respiratory diseases during pregnancy may also affect fetal brain development.
The Danish data suggest that illness could account for roughly half of the 1.9% wage gap between first- and second-born siblings. Parental behaviour may explain the rest.
As younger brothers and sisters often grumble, first-borns get more attention. American time-use data show that, throughout their childhood, they enjoy 20-30 more minutes of quality time per day than second-borns at the same age.
Researchers reckon that parents try to split their attention evenly across their children at any given point—meaning that first-borns end up with more across their childhood because subsequent babies require lots of care.
For the eldest, that may mean more brain-boosting stimulation in the crucial early years.
Younger brothers and sisters, it turns out, are not just whining.
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