What Is the Relative Age Effect?
The relative age effect is the phenomenon in which children born near the end of the school year have systematically lower academic, athletic, and career outcomes than classmates born early in the year. The mechanism lies not in the birth date but in the fact that schools and sports federations group children by a fixed cutoff (usually the calendar year or school year), so that within one group there can be children almost 12 months apart in age. This age gap creates differences in physical ability, psychological maturity, and experience, which then affect how adults evaluate children and give them opportunities.
The concept is not new. Since the 1980s, sports psychologists have noticed that in youth teams, birth dates are not evenly distributed but cluster toward the start of the year. The 2001 review by Jochen Musch and Simon Grondin in the journal Developmental Review systematized the evidence and named the phenomenon a form of "unfair competition" that hinders individual development.
Why Is Football the Most Visible Case?
Football is an ideal setting for observing the relative age effect because youth leagues group players very strictly by birth year. Research by Jochen Musch and Roy Hay in 1999 in the Sociology of Sport Journal compared player data across several countries and found a repeating pattern: professional players in Germany, Japan, Brazil, and Australia cluster among those born early in the year. Notably, when Australia changed its age-group cutoff, the birth-date peak in the data shifted to the new cutoff. This is strong evidence that the cause lies in the organizational cutoff, not in the birth month itself.
Musch and Hay's research also showed that the effect is stronger in younger age groups and tapers off as players mature, because by then physical differences matter less. However, the initial disadvantage was enough to push many children out of the training system before they had a chance to prove their ability.
At School, How Far Does This Disadvantage Extend?
Research by Kelly Bedard and Elizabeth Dhuey in 2006 in the Quarterly Journal of Economics analyzed TIMSS data from many countries and found that the youngest students in a class scored significantly lower than their classmates. The worrying part is that this disadvantage does not disappear as children grow up. Bedard and Dhuey noted that the gap persists into secondary school and even affects the likelihood of entering university.
Research by Juan-José Navarro, Javier García-Rubio, and Pedro R. Olivares in 2015 in PLOS ONE tested this effect in Chile1. They analyzed data from more than 15,000 eighth-grade students and found that the youngest students in a class had lower average scores. However, the study also found an interesting point: in Chile, parents could decide to enroll a child born early in the year one year ahead, and this group had lower results than the group born later. This shows that the disadvantage comes not only from being the youngest in the class but also from being pushed into an environment that does not match the child's developmental maturity.
All the Way to the CEO Chair?
Research by Qianqian Du and colleagues in 2012 in Economics Letters extended this effect beyond school and sports. They analyzed data on CEOs in the United States and found that those born in June and July, meaning the youngest in their class under the U.S. school entry cutoff, appeared far less often than those born early in the year. This result suggests that the disadvantage of relative age can accumulate over decades and affect even the highest leadership positions.
It should be noted that this is data from the United States and cannot be used to infer specific numbers for Vietnam. But the general pattern, that the youngest in a group has fewer opportunities to reach high positions, has been repeated in many countries and many different fields.
Summary Table of Main Studies
| Study | Data Scope | Main Finding |
|---|---|---|
| Musch & Hay (1999), Sociology of Sport Journal | Professional players in Germany, Japan, Brazil, Australia | Players cluster among those born early in the year; when Australia changed the cutoff, the birth-date peak shifted accordingly |
| Musch & Grondin (2001), Developmental Review | Review of many studies on sports | The relative age effect is a form of unfair competition that hinders individual development |
| Bedard & Dhuey (2006), Quarterly Journal of Economics | TIMSS data from many countries | The youngest students in a class score lower; the disadvantage persists into secondary school and university |
| Navarro, García-Rubio & Olivares (2015), PLOS ONE | More than 15,000 eighth-grade students in Chile | The youngest students in a class have lower scores; the group that entered school early also faced disadvantages |
| Du and colleagues (2012), Economics Letters | CEOs in the United States | Those born in June and July (the youngest in class) appear far less often |
What About Vietnam?
In Vietnam, children enter first grade by calendar year of birth. This means children born late in the year, meaning November and December, will be the youngest in the class, while children born in January and February are the oldest. This is an organizational cutoff that could create a relative age effect similar to that in other countries.
However, in the current reference list, there is no research on the relative age effect in Vietnam. This is a gap that needs to be stated clearly. Numbers from studies in Chile or the United States cannot be used to infer outcomes for Vietnam, because school entry cutoffs, curricula, and educational cultures differ from country to country. What parents should know is that this mechanism may exist, and if their child is born late in the year, they should monitor the child more closely rather than worry about star signs.
Distinguishing "There Is a Correlation" from "the Chart Is Right"
The relative age effect is a good example for distinguishing two things that are often confused. On one hand, there is a real correlation between birth month and success: people born early in the year tend to succeed more in some fields. But this correlation does not come from the birth month itself or from the zodiac sign. It comes from a clear, measurable social mechanism, and more importantly, one that can be corrected by changing the grouping cutoff or by supporting younger children.
If an astrological system predicts that people born in January are more successful than people born in December, and the data in fact show that, the prediction is still wrong for the wrong reason. It is like a clock that shows the right time twice a day: the result may coincide, but the mechanism is unrelated. This is an important point when evaluating any prediction system based on birth date.
Conclusion
Birth month does have a real effect on opportunity, but through a clear and correctable social mechanism. The disadvantage of being the youngest in a class is not a fixed fate. Studies show that when the grouping cutoff changes, the effect changes too. This means parents and schools can intervene: monitor children born late in the year closely, create opportunities for children to develop at their own pace, and avoid pushing children into environments that are too demanding. This is an evidence-based approach, far more useful than choosing a birth month by star.
