Why an autumn birthday could win you first place on sports day

Born in the right month. longwayround, CC BY-NC-ND

The playing field may not be level for school sports due to physical differences between children born in different months.

Our research measured 8,550 children’s stamina, muscle strength and power. Those born in the autumn had a clear physical advantage over children born at other times of the year.

Unfortunately, long ago in England, we adopted a September 1 cut-off for school entry. This was historically to allow children to help with the harvest. Now, it means the oldest children in each year are also those who benefit from summer vitamin D exposure – a double advantage. If we moved the cut-off to April fool’s day the smaller and less fit children would be the oldest in their school-year which might cancel out the autumn advantage.

The tests we used in our research are standard indicators of physical performance. We tested children during PE lessons, following a standardised procedure, but then we converted them to age-related percentile scores using a commercially available testing system.

Stamina was measured using the 20m shuttle-run (bleep-test) in which the children ran as the speed gets faster-and-faster until they choose to stop. Global data from this test show us children’s fitness levels are declining about 4% per decade – but we found the rate was closer to 10% in English children.

We also measured children’s strength using an handgrip dynamometer and their explosive lower body power via a timing “mat” which measures how long they are off the ground, also known as their flight time. Then, with some maths involving mass and gravity we calculated how powerful they are.

September is the best month

Media coverage of our research has omitted the important point that we controlled for the age of participants. This is important as it allows us to compare children of exactly the same age (we subtracted the date-of-test from date-of-birth to be accurate to the day). This means we did not look at the “relative age” effect (being older in your school year), and we didn’t compare by school year, only by age.

Previous research we published on relative age last year showed a small advantage for autumn and winter-born children’s fitness level depending on when they were born, but the effect was nearly twice as big as our study on the month of birth. If we were studying the relative age effect, children born in September would be the fittest.

Other researchers have found that autumn-born babies are longer (crown-rump length), heavier and more muscular at birth than summer babies. They have put this down to greater vitamin D exposure during the third trimester of the pregnancy. It is in this third trimester that the foetus develops much of its bone mass and musculature. This is why October babies whose mums get sunshine in July, August and September are bigger – and possibly why they go on to be fitter and stronger.

Born in April? No wonder you’re slower. Nick Potts/PA Archive

Finding ways to mitigate the disadvantage isn’t just important for sports. The chief medical officer, Liam Donaldson has proposed national fitness assessments for health monitoring purposes. I endorse this, but it has to be done properly, sensitively and accounting for relative age.

We used a health-related cut-offs for our tests to identify children with “clinically” low fitness. April-born children were twice as likely to have clinically low muscle strength and were 50% more likely to have low fitness. Such differences should be accounted for in any testing carried out in schools.

Ways to level the playing field

It’s unlikely the government will switch the school entry date based on differences in physical performance. However, in New Zealand (where the effect is reversed) they put weight limits in schools sports to take away the simple advantage of being the biggest. In Scotland, a small number of players in (for example) Year 8 are allowed to play for Year 7 teams.

More complex ideas could involve limiting the number of players in a team to 50% born between September and March and 50% born between April to August. Schools could even have two teams per year based on six-monthly age grouping (the under 12’s and the under 11 and a halves).

Of course this would mean some of the best and biggest children not being picked, but if sports is about talent development and potential that should be ok. That said, I would find it hard not to field the team most likely to win and imagine most teacher and coaches would, and parents of November born superstars sitting on the side-line might also query this.

One unexpected pattern in the data was that children born in October and November came from families which were, on average slightly richer than those of children born in other months. This might be that some better-off families are actually planning their family birth dates to give their children a better start by being born early in the school year. But that’s just an idea and we can’t prove it!