What would you say determines the completion of university studies? We know that young people from upper social classes are more likely to attend university than those from lower backgrounds, even when their performance in earlier stages of education is the same. We also know that women are more likely to finish a university degree, as are people born in Spain compared with those born abroad. Factors such as parents’ divorce, family size, or birth order also play a role.

Today I’m going to talk about another, perhaps less well-known factor: month of birth. This effect has nothing to do with the month itself (or with zodiac signs). The key lies in the student’s age when starting compulsory education.

Month of Birth and Age at School Entry

In Spain, the rule for entering the school system is simple: all students begin primary school in September of the academic year in which they turn six. The school cohorts are therefore organized with January 1 as the cut-off date, coinciding with the calendar year.

Unlike in many other countries, this rule is implemented strictly in Spain: delaying (so-called redshirting) or bringing forward (greenshirting) school entry is not allowed. As a result, a child born on December 31, 2025 will start primary school in September 2031, at 5 years, 8 months, and 1 day old. Meanwhile, someone born just one day later, on January 1, 2026, will start in September 2032 at 6 years and 8 months old. Just one day’s difference in date of birth translates into almost a full year’s difference in the age at which both pupils begin primary school.

That extra year represents 17.6% of the lifetime of the child born on December 31 at the moment of starting compulsory schooling, a significantly lower maturity level. It is therefore clear that being born just after the cut-off date confers a major educational advantage, at least in the early years of schooling.

Postponing Births?

Since school entry in Spain cannot be delayed or advanced once a child is born, their educational cohort is determined at birth. This creates a strong incentive for parents aware of this effect and who happen to be due around January 1 to try to delay delivery slightly, doesn’t it? That way they ensure their child enjoys the advantage of being older within their cohort.

There is evidence of this behavior in countries like Japan, South Korea, China, Norway, and Argentina. In Spain, by contrast, parents do not advance or postpone births in response to the school-entry cut-off date, although they do so to take advantage of other public policies [1].

The Effect of Month of Birth on School Performance

It is obvious that being a year older at the start of primary school makes school life easier. That difference amounts to about one-third of a standard deviation in test scores by fourth grade [2]. The effect is large and remarkably consistent across countries.

Again, the effect has nothing to do with the birth month itself. In Spain, January-born students outperform December-born ones because the cut-off is January 1. In the UK, the advantage belongs to those born in September over August (cut-off September 1), and in Japan, to April-born students over March-born ones (cut-off April 2).

Does the Effect Fade with Age?

You might reasonably think that, however evident the maturity gap is at the beginning of primary school, it must fade as students grow, reducing performance gaps. That is true, but with some qualifications.

First, the equalization of performance does not happen the same way for boys and girls. By the eighth grade (roughly age 13), the month-of-birth effect disappears among Spanish girls but persists among boys [2].

Second, early academic struggles can affect other variables before maturity levels converge. For example, when fifth graders’ willingness to exert effort is measured experimentally, January-born pupils show a much higher willingness than those born in December [3]. Likewise, parents adjust their expectations about whether their children will complete university studies according to these early differences in performance caused by birth month [4]. A lower inclination to exert effort and less ambitious parental expectations can prolong the month-of-birth effect even once maturity levels have evened out.

Third, if the effect lasts long enough, students start making educational decisions before it disappears, such as dropping out of school after reaching the school-leaving age [5]. As December-born students are more likely to leave school early, that will leave an impact when examining the month-of-birth effect on final educational attainment.

Month of Birth and University Studies

Effect on the Probability of Completing University

Although students’ maturity levels converge well before university, the month-of-birth effect can still be observed in the probability of completing a degree.

How can we check? We can analyze census microdata, for instance. If you request individuals’ exact birth dates from the Spanish Statistical Office, you can compare the likelihood of completing a university degree for people born just before and just after January 1 in any given year. Do this for several years and then average the results.

You’ll see that men born in early January are 2.5 percentage points more likely to graduate from university than those born in late December. Among women, there is no effect [6].

Effect on Field of Study

You can also examine whether there are differences in completing university studies in STEM (science, technology, engineering, and mathematics). Here, you’ll find that men born in January are 6 percentage points less likely to finish a STEM degree than those born in December. Again, there is no effect among women [6].

So, being born in early January rather than late December increases the chance of graduating from university but reduces the likelihood of completing the degree in STEM.

How to interpret these findings together? One explanation is that relative age at school entry matters most for students in the lower part of the performance distribution. For some December-born pupils, the relative-age disadvantage consolidates their poor results, making university completion impossible. Being born in January, by contrast, gives some of those students just enough of a boost to access higher education, for which they enroll in fields perceived as more accessible to maximize the chance of success.

Concluding remarks

I hope I’ve convinced you that this issue is far from trivial, and that the month-of-birth effect can be seen not only in the early years of primary school but also in higher education. Research has even documented the effect in the composition of the U.S. Congress and among CEOs of large American companies.

So, what could we do to counter this effect? First, resist the temptation to delay school entry by a year. If, instead of starting in the year children turn six, they start at seven, the problem remains: those born just before January 1 will be 6 years 8 months at entry, and those born just after, 7 years 8 months. Although the relative maturity gap is smaller at age seven, the gain is minimal. The same applies if you shift the cut-off to a later month, like September instead of January.

Resist, too, the temptation to set different entry rules for boys and girls, forcing the former to start later. That wouldn’t solve anything because the month-of-birth effect is similar for both sexes in the early years. To close the gender gap, we must address the mechanisms that prolong the effect among boys while it disappears among girls.

Relaxing the Entry Rule?

More promising proposals include making school-entry rules more flexible, allowing a one-year delay if a child’s developmental readiness is not sufficient.

Currently, how do we address in Spain the problem of forcing pupils into primary school even though they’re developmentally ready? We make them repeat second grade. That explains the large gap in second grade repetition rates between January- and December-born pupils. We return those relatively younger students to the “appropriate” cohort by making they retake that grade, but with the stigma of having repeated a year. Wouldn’t it be better to identify such cases before they start primary school and simply delay their entry?

That said, if we introduce flexibility, we should avoid leaving the decision entirely to parents. We know that mainly middle- and upper-class families (who are better informed) would take advantage of it. Yet 95% of children in Spain attend the second cycle of pre-primary education, so teachers could offer families an assessment at the end of that stage about whether delaying entry would be beneficial.

Other Proposals

Given how difficult it is to eliminate maturity gaps within a single cohort, some suggest giving extra weight to the performance of students disadvantaged by their birth month, at least in the early grades.

Perhaps a better solution would be to develop mentoring and support programs for the youngest students in each cohort. This additional help would offset developmental deficits until they disappear, preventing them from becoming learning gaps.

Moreover, it’s essential that teachers’ training programs incorporate the growing body of academic research on this topic. Teachers are well aware of the maturity gap between January and December pupils in pre-primary and primary school, but most are unaware of the long-term consequences of those early differences. Understanding the real impact of month of birth is key to designing effective classroom strategies for managing this heterogeneity.

The truth is that, as things stand, the “birth-month lottery” disadvantages some students and benefits others, leaving a lasting imprint on their academic lives. I hope this post helps to make the problem visible and encourages measures to reduce the long-term impact of birth month.

[1] Valdés, M. T., & Requena, M. (2023). School-Entry Cut-off Date and Birth Timing: No Evidence of Shifting in Spain. Population, 78(2), 281–296. https://doi.org/10.3917/popu.2302.0281

[2] Valdés, M. T. (2024). The effect of the month of birth on academic achievement: Heterogeneity by social origin and gender. European Societies, 26, 1094–1120. https://doi.org/10.1080/14616696.2023.2289652

[3] Radl, J., & Valdés, M. T. (2024). Month of Birth and Cognitive Effort: A Laboratory Study of the Relative Age Effect among Fifth Graders. Social Forces, 103(1), 153–172. https://doi.org/10.1093/SF/SOAE023

[4] Bernardi, F., & Valdés, M. T. (2025). Month of Birth, Early Academic Achievement, and Parental Expectations of University Completion: A New Test on Sticky Expectations. Sociology of Education, 98(1), 44–61. https://doi.org/10.1177/00380407241300306

[5] Requena, M., & Valdés, M. T. (2024). Are Younger Students More Likely to Drop Out? Spanish Evidence on the Moderator Role of Gender and Social Origin. European Journal of Education, 59(4), e12753. https://doi.org/10.1111/EJED.12753

[6] Valdés, M. T., & Requena, M. (2024). The effect of the age at school entry on educational attainment and field of study: An analysis using the Spanish census. Higher Education, 87, 1061–1083. https://doi.org/10.1007/S10734-023-01053-5