But 12.5% of them were born in March.
How can four months make such a profound difference?
You can blame what academics call the "relative age effect," in which "an initial advantage attributable to age gets turned into a more profound advantage over time," according to New York magazine.
You see it most in the way kids are grouped by age in school. If you were born in June or July, so the logic goes, you'll be the youngest in your class. And if you're born in March or April, you're one of the oldest.
"Older children within the same grade tend to do better than the youngest, who are less intellectually developed," said study coauthor and University of British Columbia finance professor Maurice Levi. "Early success is often rewarded with leadership roles and enriched learning opportunities, leading to future advantages that are magnified throughout life."
Further research has supported the finding, like a Duke University working paper that tracked North Carolina students for academic success and delinquent behavior.
"[Students] born just after the cut date for starting school are likely to outperform those born just before in reading and math in middle school, and are less likely to be involved in juvenile delinquency. On the other hand, those born after the cut date are more likely to drop out of high school before graduation and commit a felony offense by age 19. We also present suggestive evidence that the higher dropout rate is due to the fact that youths born after the cut date have longer exposure to the legal possibility of dropping out."
It's even more staggering when you see the success rates illustrated graphically.
Here are the reading scores: 
And the math scores:
That same birthday effect has been spotted in athletics, most famously by Malcolm Gladwell is his account of pro hockey players in "Outliers." The pop psych writer argues that since the eligibility cutoff for junior hockey in Canada is January 1, "a boy who turns 10 on January 2, then, could be playing alongside someone who doesn't turn 10 until the end of the year."
While clearly your birthdate isn't your sole determiner of success, it may be frustrating to see that such a simple, random thing such as the month that you're born in creates cascading advantages or disadvantages through life.
"Our study adds to the growing evidence that the way our education system groups students by age impacts their lifelong success," said Levi. "We could be excluding some of the business world's best talent simply by enrolling them in school too early."
Some parents have tried to side-step this bias by way of "academic redshirting," where you give the child another year in pre-school so that she starts kindergarten with an advantage in maturity.
But that may be a terrible idea, too: the preliminary research suggests that redshirted kids have lower IQs and life-time earnings compared to their peers.