Do HDI and Median age go together across countries?

Across 187 countries the link before adjusting for anything is r = +0.88, on a scale from -1 (opposite) to +1 (in step). A version that uses only each country's rank gives +0.90.

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

Comparing each country only with itself across 1990-2023, and setting aside what happened worldwide in each year, the link is +0.033 (192 countries, 5,938 country-years). Using year-to-year changes, it is +0.047.

Does HDI cause Median age?

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.

HDI vs. Median age

Holds up after adjusting for income

Across 187 countries, HDI and Median age rise and fall together at r = +0.88 (on a scale from −1 to +1, where 0 means no link). Comparing only countries with similar income (GDP per person) shrinks it to +0.45 (q = 0.043, allowing for the many pairs tested). Much of the first figure is that both measures rise with national income, but a moderate positive link remains.

Comparing countries across the middle half of median age, 21 to 37.8 years, hdi is better by 0.237 points at the top of that range than the bottom. Comparing only countries with similar national income, the gap is 0.0692 points.

Link before adjusting (r)+0.884p = 6.8e-63 · 187 countries

After adjusting for national income+0.453p = 8.8e-11 · likely range [+0.33, +0.56]

p, allowing for regions0.001standard error 0.076 · 7 regions

Check for luck (q)0.043across 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.

HDI · index, 0 to 1link with income +0.96 · proxy

Median age · yearslink with income +0.86 · proxy

Both of these rise and fall closely with national income, so a strong link between them before adjusting was close to certain. Read the adjusted figure, not the first one.

“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	+0.884	6.8e-63	187
By rank (Spearman), not adjusted	+0.900	1.6e-68	187
Standard (Pearson), adjusted for national income	+0.453	8.8e-11	187
By rank (Spearman), adjusted for national income	+0.473	9.5e-12	187

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 187 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: +0.431 (a minus sign means it grew).

How solid is it?

Leaving out one country at a time, r ranges from +0.411 (without GUY) to +0.474 (without TON). 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: GUY (-0.041).

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	30	+0.30
Europe & Central Asia	50	+0.24
Latin America & Caribbean	31	+0.62
Middle East, North Africa, Afghanistan & Pakistan	22	+0.21
Sub-Saharan Africa	46	+0.46

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	63	+0.36
Middle third	62	+0.50
Richest third	62	+0.55

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	108	+0.53
Muslims	49	+0.46

By last european coloniser

	n	r
France	25	+0.51
Never a European colony	21	+0.41
Spain	20	+0.76
United Kingdom	63	+0.44

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 = +0.453 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 34years. 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 = +0.033, 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 hdi moved did not tend to see their median age move with it.

Across countries (adjusted)+0.453for comparison

Within countries+0.785each country vs itself

…and vs the same year elsewhere+0.033q = 0.761 · 192 countries

Year-on-year changes+0.047q = 0.260

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.
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.

5,938 country-years (one country in one year), 192 countries, 1990–2023, about 31 years each on average. Share of the variation that happens within countries over time: 8% for HDI, 9% for Median age. 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): +0.033. 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.

This curve peaks at the edge of the window (lag -10), so it is still drifting where the search stops. That pattern usually reflects a slow trend left over in the data, not a link with real timing. The peak value should not be read as the “true” link at the right lag.

The data

	index, 0 to 1	years	Years
Iceland ISL	0.972	35.8	2023
Norway NOR	0.97	39.5	2023
Switzerland CHE	0.97	42.3	2023
Denmark DNK	0.962	41.2	2023
Sweden SWE	0.959	39.9	2023
Germany DEU	0.959	45.1	2023
Australia AUS	0.958	37.8	2023
Netherlands NLD	0.955	41.4	2023
Hong Kong SAR, China HKG	0.955	46.2	2023
Belgium BEL	0.951	41.4	2023
Ireland IRL	0.949	38.4	2023
Finland FIN	0.948	42.8	2023
Singapore SGP	0.946	35.1	2023
United Kingdom GBR	0.946	39.8	2023
United Arab Emirates ARE	0.94	31.2	2023

United Nations Development Programme, Human Development Report 2025
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

Sources

HDI

Human Development Index

United Nations Development Programme, Human Development Report Office · via Our World in Data · CC BY 3.0 IGO · updated annual

The geometric mean (a kind of average that multiplies the parts together) of three parts: income per person (GNI per capita), life expectancy, and schooling (expected years and average years). All four inputs are on this site, and each is flagged. So a link between the HDI and any of them is shown for what it is: the index matching a part of itself. For the same reason it is kept out of this site's own topic scores. Those already contain its parts, and adding it would count each twice. It is the most-cited development index there is. It is here mostly so its parts can be read separately.

Median age

Median age of the population

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

The age that splits a population in half: half are older and half are younger. The shares aged under 15 and over 65 are already on the site. All three are treated as measuring the same thing. They are readings of the same age pyramid, so they are bound to match, and their agreement is not a finding. No "better" direction is set.

How this was computed

Data used: most recent value in 2010-2025; values come from 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 7 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 HDI and Median age

This page shows that two measures go together. It does not show that HDI changes Median age, or the reverse. These figures also describe countries, not individual people. See the ecological fallacy.