How Your Birth Date Can Shape Your Life
Jesse and Sam both start kindergarten this year. Jesse was born on September 4th, 2014. Sam was born August 24th, 2015. Jesse is roughly 20% older than Sam on the day they start school. While this relative difference in maturity will dwindle as Jesse and Sam grow up, studies suggest that it could give Jesse an advantage in sports, academics, and even politics that will persist for the rest of their lives.
This is the relative age effect. It's spawned from the creation of cohorts, particularly for school and sports. Since human births span fairly evenly across a calendar year, it means that some kids in – for example – sixth grade or a ten-year-old sports team will be older than their peers. Due to this age disparity, they might be stronger and faster, with more developed brains. These advantages can result in better athletic and academic performance early on, which can lead to additional benefits. Relatively older kids might get promoted to better sports teams or placed into "gifted" academic programs. What started as a simple difference in age can snowball into better life outcomes compared to their younger peers, even as the relative age difference disappears.
A 2006 study looked at what this means academically.
"The youngest members of each cohort score 4–12 percentiles lower than the oldest members in grade four and 2–9 percentiles lower in grade eight," economists at UC-Santa Barbara reported. "In fact, data from Canada and the United States show that the youngest members of each cohort are even less likely to attend university."
Two years later, researchers discovered that the oldest students within a grade level are 4 to 11% more likely to attain leadership positions in high school like sports team captain, club president, or school treasurer.
Incredibly, the relative age effect is even apparent in the United States Congress.
Nowhere is the effect more obvious, however, than in professional sports. A 2013 report found that "36% of players drafted by National Hockey League teams between 1980 and 2007 were born in the first quarter of those years, or from January to March, compared to 14.5% of draftees who were born in the fourth quarter."
The explanation for such a pronounced effect is simple: due to their advantage in maturity, relatively older kids are perceived to be more talented and are thus granted more play time, given more attention, and steered to more elite teams.
While the relative age effect is real, it by no means dictates the life of any individual. Developmental differences due to environmental and genetic factors surely dwarf the affect of age disparities. Dedicated parenting, decent schooling, a healthy social life, and proper nutrition will do far more for a child than simply being slightly older than their peers. On the other hand, if you really want your progeny to play professional sports, being born in January just might give them an edge...