Stoking the fires of sibling rivalry doesn't take much, but the results of the biggest study in history into birth order and personality won't do anything to help matters, revealing as it does that first-born children have a higher IQ than those born later. Ouch.

Researchers at the University of Illinois used a sample of 377,000 schoolchildren and found there were also differences in personality traits, with the eldest sibling tending to be more extroverted, agreeable and conscientious.

Which sounds like bad news for those of us born second or third, but before you use it to shore up your middle child syndrome, bear in mind the IQ difference is only one point - and Dr Rodica Damian, postdoctoral researcher and professor of psychology at the University of Houston, described it as "statistically significant but meaningless".

Though the research also found such differences as first-born children being less likely to have anxiety than their siblings, Dr Rodica said they were "infinitesimally small" and parents shouldn't worry too much.

It's official: eldest siblings are smarter, nicer and more outgoing