Science has just published a study of almost a quarter-of-million people providing strong evidence that oldest children have slightly higher IQs, and, most interestingly, the evidence suggests that this isn’t a biological effect – it’s likely to do with family environment and upbringing.
In fact, first-born children are known to have a number of psychological differences. For example, they are less likely to be gay, show differences in autistic-like traits, and are typically less severely affected by schizophrenia if it occurs.
These differences have often been explained by a theory that argues that the mother adapts her immune system during the first pregnancy and it might not be fully attuned to later children and this might affect the brain development of subsequent children.
In order to test this idea the Science study looked at the records of almost 250,000 Norwegian army recruits, all of which have routine IQ tests and full medical and family histories.
It turned out, as has been found many times before, that first-born children had higher IQs by about 3 points on average.
Crucially, it also turned out that some second-born children who had an older sibling who had died young also had higher IQs.
In other words, although they were second-born biologically, they were brought up as the oldest child after their sibling passed away.
Being brought up as the oldest child seems to be the crucial factor: family-rank, not birth order affects IQ. This suggests that the immune system theory is unlikely to explain this effect.
This has generated a great deal of discussion and many parents are interested in whether they can provide the ‘first child advantage’ to their younger children as well.
Some psychologists are suggesting that the effect might be because older children get the chance to coach the junior family members which may help them consolidate knowledge and provide practice in manipulating information.
It’s also interesting that a recent study on birth-order in Thai medical students found exactly the reverse pattern. Younger siblings were found to be more intelligent and have more positive personality factors.
All of these studies suggest that culture and environment are crucial factors during childhood, both for mental and emotional development.