Birth Month Bias in Youth Track and Field

Many people think talent shows up in the teen years. But in Norwegian track and field, the gap starts much earlier. At age ten, the youngest age allowed to compete, kids born in the first three months of the year made up 28.3 percent of all competitors. Those born in the last three months made up only 19.7 percent. In the general population, those groups are closer to 24.6 percent and 22.9 percent. That means the oldest kids in the age group show up far more often right from the start.

Researchers looked at every single athlete in the Norwegian Athletics Federation from 2011 to 2026. That is 87,624 people and more than 1.3 million performances. They compared birth dates against official population numbers from Statistics Norway. The goal was to see if being born early in the year gives a lasting edge or if the edge grows as kids get older.

Some records were missing birth dates, especially for the youngest athletes. About 61 percent of ten to twelve year olds had no birth date on file. Most of these kids quit after a very short time. The researchers used special statistical methods to account for these gaps so the missing information would not trick the results.

The advantage did not snowball through the teenage years. The ratio of first quarter births to fourth quarter births stayed between 1.36 and 1.75 all the way to age 25. For girls, there was no sign that the gap grew with age. For boys, there was a small bump around ages 18 and 19, but nothing like a steady climb. Even when looking only at ages 13 to 25, where birth records were almost complete, the story stayed the same.

The pattern points to an early starting line. The oldest children in the group get picked first. Clubs see them at sign up and choose them right away. Changing things later, during teen talent searches, misses the point. The fix has to happen at the door, not years down the road.