Do Age at childbearing and PISA science go together across countries?

Across 87 countries the link before adjusting for anything is r = +0.66, on a scale from -1 (opposite) to +1 (in step). A version that uses only each country's rank gives +0.66.

Does the link hold within countries over time, not just across countries?

Comparing each country only with itself across 2006-2022, and setting aside what happened worldwide in each year, the link is -0.113 (56 countries, 330 country-years). Using year-to-year changes, it is —.

Does Age at childbearing cause PISA science?

This analysis cannot answer that. It shows only that the two go together. A link between two country measures, adjusted or not, does not show that either one changes the other. Country figures also say nothing about individual people.

Age at childbearing vs. PISA science

Fades after adjusting for income

Across 87 countries, Age at childbearing and PISA science rise and fall together at r = +0.66 (on a scale from −1 to +1, where 0 means no link). Comparing only countries with similar income (GDP per person) shrinks it to +0.28, which could easily be chance (q = 0.30, allowing for the many pairs tested, and allowing for countries in the same region being alike). So the first figure mostly shows richer countries differing from poorer ones, not a link between these two things.

Comparing countries across the middle half of pisa science, 403 to 491 PISA points, age at childbearing is 1.82 years higher at the top of that range than the bottom. Comparing only countries with similar national income, the gap is 0.896 years.

Link before adjusting (r)+0.664p = 2.4e-12 · 87 countries

After adjusting for national income+0.284p = 0.008 · likely range [+0.08, +0.47]

p, allowing for regions0.030standard error 0.086 · 5 regions

Check for luck (q)0.304across 12,497 pairs

How to read these.r runs from −1 (one goes up as the other goes down) to +1 (they rise together), and 0 means no link. Adjusting for national income means comparing countries with the same GDP per person, so differences in wealth can’t explain the link. Income is used in percentage terms, since an extra $1,000 means more to a poor country than to a rich one. p is how likely a link this strong would be if there were really none. Below 0.05 means it is unlikely to be chance. Very small values are written in short form: 1.1e-32 means a decimal point followed by 31 zeros and then 11, so very close to zero. The likely range is where the true figure probably lies, and a range that crosses zero means the link could easily be chance. Countries in the same region tend to resemble each other, so the third figure allows for that, which gives a fairer p. The standard error is the typical size of the error in r. Finally, testing this many pairs means some will look strong by luck alone. A q below 0.05 means this link is unlikely to be one of those.

Age at childbearing · yearslink with income +0.44 · moderate

PISA science · PISA pointslink with income +0.78 · high

“Link with income” is how closely each measure tracks GDP per person across countries, from −1 to +1.

Every country as a dot

best-fit line line after adjusting

Four ways to measure the link

Method	r	p	Countries
Standard (Pearson), not adjusted	+0.664	2.4e-12	87
By rank (Spearman), not adjusted	+0.662	3.0e-12	87
Standard (Pearson), adjusted for national income	+0.284	0.008	87
By rank (Spearman), adjusted for national income	+0.206	0.058	87

The rank version uses only each country’s place in line, from lowest to highest, so a few extreme countries count for less. The adjusted and unadjusted versions use the same 87 countries: those that report both measures and GDP per person. So the gap between them comes from the adjustment, not from a change in which countries are counted. Drop in the size of r after adjusting: +0.380 (a minus sign means it grew).

How solid is it?

Leaving out one country at a time, r ranges from +0.249 (without PAN) to +0.328 (without VNM). The bar above shows that range on a scale from −1 to +1, with the solid line at the full result. The single country that moves r most: VNM (+0.044).

Within each region

Countries are not separate, unrelated cases, and neighbors share a lot. A link that holds inside each region is far more believable than one that shows up only when comparing regions.

	n	r
East Asia & Pacific	15	+0.17
Europe & Central Asia	44	+0.09
Latin America & Caribbean	15	+0.53

By income third

If the link shows up only in the richest third of countries, it is a feature of rich countries, not a general pattern.

	n	r
Poorest third	29	-0.21
Middle third	29	+0.43
Richest third	29	+0.49

By largest religious group

Countries grouped by a label they share, such as main religion, rather than by a number. Groups of fewer than twelve countries are not shown. These groups differ in income, region and history all at once. So a gap between them is a place to start asking questions, not an answer.

	n	r
Christians	55	+0.33
Muslims	18	-0.01

By last european coloniser

	n	r
Spain	14	+0.40
United Kingdom	14	+0.08

Without outliers (countries far outside the middle half of countries on either measure, by more than one and a half times the width of that middle half): r = +0.284 after dropping 0 countries.

Two series, one country

Both measures over the years for one country you pick. The two charts are stacked so they share only the years along the bottom.

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Over time, within countries

Everything above compares countries with each other at one moment. This section compares each country only with itself over 17years. So a link here can’t come from anything about a country that stays the same, such as its history, geography, institutions or culture.

Within countries over time, there is no link that is clearly more than chance (r = -0.113, comparing each country only with itself over the years, and each year only with the same year elsewhere). Whatever the cross-country picture shows, countries whose age at childbearing moved did not tend to see their pisa science move with it.

Across countries (adjusted)+0.284for comparison

Within countries-0.216each country vs itself

…and vs the same year elsewhere-0.113q = 0.438 · 56 countries

Year-on-year changes—q = —

Read with care.

Almost all of this indicator's variation is between countries, not within them, so the within-country estimate rests on a small part of the data.

330 country-years (one country in one year), 56 countries, 2006–2022, about 6 years each on average. Share of the variation that happens within countries over time: 5% for Age at childbearing, 15% for PISA science. The margins of error allow for each country appearing many times, and for each year’s value being close to the year before.

Year-to-year changes are a cautious test, not a final answer. Looking only at changes from one year to the next removes each country’s own trends. But it also makes measurement mistakes louder, which pulls the results toward zero. A near-zero result here is weak evidence of no link, not proof.

With one measure years behind the other

Same year (lag 0): -0.113. A peak away from zero does not show which comes first. These series change slowly — this year’s value is close to last year’s. That makes the curve smooth and nearly symmetric, whatever produced the data. Even a sharp peak at one lag (a gap of some years) would not prove that one measure changes the other.

The largest value on this curve (-0.24 at lag +5) is higher than the same-year one by 0.13. That gap is not a finding. It is the largest of 16 closely related estimates, and picking the biggest of many tests always makes it look bigger. That is exactly why nothing on this site is ranked or chosen by lag.

The data

	years	PISA points	Years
Korea, Rep. KOR	33.4	528	2023, 2022
Hong Kong SAR, China HKG	32.9	520	2023, 2022
Ireland IRL	32.7	504	2023, 2022
Spain ESP	32.5	485	2023, 2022
Luxembourg LUX	32.3	477	2023, 2018
Italy ITA	32.3	477	2023, 2022
Switzerland CHE	32.3	503	2023, 2022
Singapore SGP	32.3	561	2023, 2022
Macao SAR, China MAC	32.1	543	2023, 2022
Greece GRC	32	441	2023, 2022
Japan JPN	31.9	547	2023, 2022
United Arab Emirates ARE	31.9	432	2023, 2022
Netherlands NLD	31.8	488	2023, 2022
Portugal PRT	31.7	484	2023, 2022
Cyprus CYP	31.7	411	2023, 2022

United Nations, Department of Economic and Social Affairs, Population Division: World Population Prospects 2024 and Estimates and Projections of Women of Reproductive Age Who Are Married or in a Union
OECD databases as named on each indicator: PISA, Social Expenditure Database (SOCX), Income Distribution Database, Development Assistance Committee statistics, Family Database, Government at a Glance, Corporate Tax Statistics, employment and environment statistics

Sources

Age at childbearing

Mean age of mothers at childbearing

United Nations Department of Economic and Social Affairs, Population Division · via Our World in Data · CC BY 3.0 IGO · updated biennial

The average age of women giving birth in the year, based on how many births women of each age have. Later childbearing goes with fewer births, but not one for one. A country can have late but plentiful births, or early but few. No "better" direction is set.

PISA science

PISA mean science score

OECD · via Our World in Data · CC BY 4.0 (OECD terms and conditions) · updated annual, except the snapshots noted per indicator

How this was computed

Data used: most recent value in 2010-2025; values come from 2015–2023. To adjust for income, each measure is first predicted from GDP per person (in percentage terms, on a log scale) with a straight-line fit. Only what is left over is then compared. This is called a partial correlation. The line is drawn the standard way (ordinary least squares), which picks the line that comes closest to all the countries at once. The error margins allow for countries in the same region resembling each other, using 5 World Bank regions. The q-value corrects for testing all 12,497 pairs at once, using the Benjamini–Hochberg method. Underlying data last updated DYB 2024. How the country data works · explore Age at childbearing and PISA science

This page shows that two measures go together. It does not show that Age at childbearing changes PISA science, or the reverse. These figures also describe countries, not individual people. See the ecological fallacy.