Do Adolescent fertility and Married by 18 go together across countries?

Across 136 countries the link before adjusting for anything is r = +0.81, on a scale from -1 (opposite) to +1 (in step). A version that uses only each country's rank gives +0.83.

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

Comparing each country only with itself across 1990-2024, and setting aside what happened worldwide in each year, the link is +0.423 (31 countries, 204 country-years). Using year-to-year changes, it is —.

Does Adolescent fertility cause Married by 18?

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.

Adolescent fertility vs. Married by 18

Holds up after adjusting for income

Across 136 countries, Adolescent fertility and Married by 18 rise and fall together at r = +0.81 (on a scale from −1 to +1, where 0 means no link). It holds up almost unchanged when only countries with similar income (GDP per person) are compared (r = +0.69, q = 0.023, allowing for the many pairs tested).

Comparing countries across the middle half of adolescent fertility, 18.7 to 69.7 births per 1,000 women aged 15-19, married by 18 is worse by 15.2% of women aged 20-24 at the top of that range than the bottom. Comparing only countries with similar national income, the gap is 14% of women aged 20-24.

Link before adjusting (r)+0.813p = 2.4e-33 · 136 countries

After adjusting for national income+0.688p = 3.1e-20 · likely range [+0.59, +0.77]

p, allowing for regions0.001standard error 0.103 · 6 regions

Check for luck (q)0.023across 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.

Adolescent fertility · births per 1,000 women aged 15-19link with income -0.81 · proxy

Married by 18 · % of women aged 20-24link with income -0.60 · moderate

“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	+0.813	2.4e-33	136
By rank (Spearman), not adjusted	+0.829	1.1e-35	136
Standard (Pearson), adjusted for national income	+0.688	3.1e-20	136
By rank (Spearman), adjusted for national income	+0.717	1.5e-22	136

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 136 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: +0.126 (a minus sign means it grew).

How solid is it?

Leaving out one country at a time, r ranges from +0.671 (without GUY) to +0.706 (without AGO). 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: AGO (+0.018).

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	20	+0.72
Europe & Central Asia	26	+0.33
Latin America & Caribbean	25	+0.56
Middle East, North Africa, Afghanistan & Pakistan	17	+0.94
Sub-Saharan Africa	42	+0.69

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	46	+0.77
Middle third	45	+0.59
Richest third	45	+0.69

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	76	+0.64
Muslims	43	+0.88

By last european coloniser

	n	r
France	24	+0.78
Never a European colony	19	+0.33
Spain	19	+0.64
United Kingdom	43	+0.56

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 = +0.661 after dropping 5 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 35years. So a link here can’t come from anything about a country that stays the same, such as its history, geography, institutions or culture.

The link shows up within countries over time (r = +0.423) but mostly disappears in year-to-year changes (r = —). That points to slow trends particular to each country, not the two measures moving together from year to year.

Across countries (adjusted)+0.688for comparison

Within countries+0.774each country vs itself

…and vs the same year elsewhere+0.423q = 7.0e-5 · 31 countries

Year-on-year changes—q = —

Read with care.

Much of the within-country association disappears once shared global year effects are removed, which points to a common worldwide trend rather than a country-level relationship.

204 country-years (one country in one year), 31 countries, 1990–2024, about 7 years each on average. Share of the variation that happens within countries over time: 17% for Adolescent fertility, 14% for Married by 18. 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

The data

	births per 1,000 women aged 15-19	% of women aged 20-24	Years
Central African Republic CAF	161	61%	2024, 2019
Equatorial Guinea GNQ	153	29.5%	2024, 2011
Mozambique MOZ	152	48.4%	2024, 2023
Niger NER	142	76.3%	2024, 2012
Angola AGO	139	23.1%	2024
Mali MLI	136	48.5%	2024
Chad TCD	132	60.6%	2024, 2019
Madagascar MDG	129	38.8%	2024, 2021
Liberia LBR	125	24.9%	2024, 2020
Guinea GIN	117	46.5%	2024, 2018
Zambia ZMB	115	23.9%	2024
Malawi MWI	113	38.7%	2024
Tanzania TZA	113	29.1%	2024, 2022
Congo, Rep. COG	108	27.3%	2024, 2015
Cameroon CMR	105	29.8%	2024, 2018

World Bank, World Development Indicators
World Bank, World Development Indicators

Sources

Adolescent fertility

Adolescent fertility rate (births per 1,000 women ages 15-19)

UN Population Division, World Population Prospects · CC BY 4.0 · updated quarterly

Married by 18

Women who were first married by age 18 (% of women ages 20-24)

UNICEF · CC BY 4.0 · updated quarterly

From household surveys, mostly in lower-income countries, and only in years a survey ran. No year reaches sixty countries, so the latest value used is up to a decade old for some. Rich countries rarely report it at all, which shapes any link with income.

How this was computed

Data used: most recent value in 2010-2025; values come from 2010–2025. 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 6 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 Adolescent fertility and Married by 18

This page shows that two measures go together. It does not show that Adolescent fertility changes Married by 18, or the reverse. These figures also describe countries, not individual people. See the ecological fallacy.