H071
Does your birth month help decide whether you make the NHL?
Birthdays should be spread evenly through the year, about 25% in each quarter. Among NHL draftees, they aren't.
Share of NHL draftees by birth quarter
2,736 Canadian-born skaters drafted from 1980 onward
Kids born just after the age cutoff are almost a year older than their teammates, and coaches mistake that year for talent. Canadian youth hockey groups players by birth year, so a child born January 1 plays alongside one born December 31. At age eight, that extra year is about 12% more life: bigger, stronger, and better coordinated. Those kids make the top teams, get more coaching and ice time, and the head start grows. By draft day the age gap no longer matters, but the selection already happened.
Any system that compares people at one moment can mistake an early start for ability. The same pattern appears in school readiness, gifted programs, and youth soccer.
Try it somewhere else
A school picks students for its advanced reading group at the start of first grade.
Which students are most likely to be chosen, and why might that not mean they read best?
The twist at the top
The same study found that players born late in the year who did get drafted went on to play more NHL games, on average, than early-born draftees picked at the same spot. Because they had to overcome the disadvantage, the ones who made it tended to be better than their draft position suggested. Teams were, in effect, passing over talent.
Related exhibits
Sources: Robert O. Deaner, Aaron Lowen, and Stephen Cobley, "Born at the Wrong Time: Selection Bias in the NHL Draft," PLOS ONE, 2013 (N = 2,736 Canadian-born non-goaltenders). Relative age effect: Malcolm Gladwell, Outliers (2008), after Roger Barnsley et al., 1985.