You know the stereotypes. The oldest is bossy but more independent. The middle child is quiet and shy. And the baby is, well, the baby, spoiled and lazier than her older siblings. Countless books and blog posts have been written about how birth order affects personality, including plenty of them here on MNN. So much as been written, in fact, that the theory has taken on a life of its own, even shaping the way parents and other family members treat each child. But a new study turns the notion on its head, suggesting for the first time that birth order has absolutely nothing to do with the way our kids' personalities develop.

For the study, which the authors have called "the biggest in history looking at birth order and personality,” researchers from the University of Illinois at Urbana-Champaign studied 377,000 high school students to find out how their birth order affected their personality and their IQ. They found that most of what we thought we knew about birth order was just a myth.

The study did find that oldest children tended to have a higher IQ than their younger siblings. But the difference, which only amounted to about one IQ point, was considered significant from a statistical standpoint but meaningless in real life. One IQ point probably wouldn't even affect which college these kids get into, so it's hardly something that should shape their future.

As for personality traits, again the study found that firstborns were often more extroverted and responsible than their peers who were younger siblings, but this time, the difference was so slight that it was not even statistically significant. The researchers called the difference "infinitesimally small." Yes, it's there, but it's not something that could reliably be used in any way to shape or predict personality based on birth order.

The main difference between this study and previous research that has linked birth order to personality is the large sample size, which made it easier for the study's authors to get more accurate results. And while most birth order research has compared siblings within the same family, this study looked at this aspect as well as how these kids compared to other children from different families. The results were consistent throughout; birth order just did not matter.

So if birth order doesn't affect personality, what does? It could be that the psychology here has created its own conclusion. Parents have been told for so long that firstborns are smarter and more responsible than their younger peers, so they automatically treat them as such. And this perpetuates the myth and the cycle.

Other theories have suggested that your birth season may have more to do with your personality development than your birth order.

Bottom line: All kids are different. It doesn't matter whether they're born first, last, or somewhere in between — they're equally likely to be responsible or irresponsible, introverted or extroverted, bossy or lazy. Parents need to give kids a chance to let those characteristics develop without assigning them roles at birth.

Forget what you 'know' about birth order
New research upends decades' old beliefs about birth order and how it affects a child's development.