Cutoff dates for school entry are generally in the September through December time period. If one assumes a September 1 cutoff, then those entering the first year of school and born in September will be up to a year older than a child born in August—a large difference at age five.
Parents are concerned about this age effect and often hold back their younger children for a year so that they will enter as the oldest children. The authors of the referenced study apparently needed to move their months for comparison far enough away from the transition months in order to draw a conclusion.
One might be tempted to write off this conclusion as a statistical fluke, but that would be a mistake. The existence of a birth-date effect is well known in academia. The British attempted to survey all that was know about the subject and issued a report in 2009 that is commonly referred to as The Cambridge Assessment. Most attention was placed on British schools, but data from other countries was reviewed as well.
The data seems to vary from school system to school system and from country to country, but there is no doubt that such an effect exists and that it persists throughout the educational careers of the students. Some summary findings:
"In the UK, where the school year starts on September 1, the disadvantage is greatest for children born during the summer months (June, July, August). "
"The effect of being the youngest in the year group holds in other countries where the school year begins at other times in the calendar year."
This effect of birth date on achievement is most pronounced in competitive sports where age grouping is done. In his book, Outliers, Malcolm Gladwell described the effect in the context of Canadian hockey players where the high performers are generally born in the first three months after the age cutoff. Gladwell explained the effect as being caused by older, stronger, more mature children being able to perform better than their younger counterparts in the early years of competition. This enhanced performance earned them more attention, better coaching, more practice time, and more intense competition. Once players were sorted by ability, the differences in ability would propagate and persist throughout the children’s playing careers.
Gladwell warned us that sorting young children into ability groups at an early age and providing better education to the faster learners would cause a similar effect in educational accomplishment. The size of the effect would be smaller than in sports, but it would persist. The Cambridge Assessment agrees.
This is a statistically significant effect. It convinces us that the effect persists throughout education, but it doesn’t tell us how it relates to ultimate accomplishment or how it is propagated.
If one believes the sports analogy, then one would expect the effect to depend on whether or not children are subjected to ability grouping at an early age. Some countries also delay formal education until age seven. Both actions might be likely to lessen age-related effects.
The Cambridge Assessment finds evidence to support the contention that early ability grouping contributes to the performance differences.
Finland begins formal education at age seven; Denmark begins at age six. At age 13 there is no discernible birth-date effect in the children of either country. This would suggest that starting school at a greater age lessens disparities. But then there is Sweden. Sweden begins school at age seven, but it groups by ability in the early grades. Its children do demonstrate a birth-date effect. Denmark forbids ability grouping before the age of 16. Finland has a similar policy. The data from these three countries seems to indicate that separating students into groups based on perceived notions of ability can be a harmful practice.
If the birth-date effect is large enough to effect the probability of attempting a university education, that is a significant effect. If it can affect the probability that a child can become a professional hockey player, that is a significant effect. If one chooses to take the study of CEO birth dates at face value, then that is the business equivalent to making it as a professional athlete.
The CEO data then is telling us that we are doing something terribly wrong in our educational system. The Cambridge Assessment data suggests that the problem can be fixed.
So why not fix it! We have enough sources of inequality. We shouldn’t have to worry about the month in which our child was born.