When a pyramid becomes a dome…
by Nalaka Gunawardene
September 11, 2015

Question: Where does a pyramid turn into a dome, and then a barrel?

Answer: When it’s the population pyramid!

The Population Pyramid is considered the most effective way to visualize the age and sex distribution of a population.

But on-going demographic trends globally and nationally are slowly changing the familiar shape.

The pyramid shape first emerged when we drew a chart with each age group represented by a bar, and each bar ranged one above the other: youngest at the bottom, and oldest at the top. The two sexes were separately shown side by side.

Of course, it’s not a perfect pyramid as there are some variations. For example, as you go up the age structure, there are often more women than men in the older age groups. This is because women generally have a longer life expectancy.

The pyramid shape has held for much of history for most countries and populations. Most people lived considerably shorter lives than today, and death rates were high. Children and youth were the most numerous groups, and very few people survived to beyond 60.

That started changing in the mid to late 20th century: the pyramid looks more and more like a dome by now (2015). The young still have the highest head count, but as a percentage, they add up to less than they used to.

Changing shape of population age structure - Image courtesy: The Economist
Changing shape of population age structure – Image courtesy: The Economist


In another few decades, even the dome shape will be gone – and replaced by an irregularly shaped barrel. The demographic forces are such that we will never see a pyramid-like age structure again. So it’s a slow goodbye!

Sri Lanka’s population structure has broadly followed the same global trend.

Sri Lanka's population age structure 1981, 2012 and 2041 - Image from Census of Population and Housing 2012 Key Findings
Sri Lanka’s population age structure – 1981, 2012 and 2041 (Image from Census of Population and Housing 2012 Key Findings)

In 1960, it closely resembled a pyramid. But since 1970, the bottom of that pyramid has been slowly but surely shrinking. The upper levels, meanwhile, have been expanding.

The latest actual population data we have is from the Census of Population and Housing, conducted in 2012. The Key Findings publication, released in 2014, briefly mentions the changing shape of Sri Lanka’s population structure.

“The age pyramid for different years shows the changes in age composition as a result of changing births, deaths and migration patterns of the population,” it notes.

In 1981, the last time a census covering the whole island was carried out, the image was still akin to a pyramid: the broad base represented the fairly large number of children in our population.

Three decades on, by 2012, the working age population has increased in relation to the child population. Hence the dome-like shape.

The report also offers a population projection for 2041. By then, it says, “the structure is far different – with a growing number of elderly population and shifting shape of the pyramid to ‘barrel’ type”.

These changing shapes represent a demographic transition underway worldwide.

Commenting on this, The Economist magazine noted in November 2014: “In 2015 demographers, teachers and politicians will stop talking about the population pyramid and start referring to the population dome. The change in terminology will reflect a profound shift in the shape and structure of societies—a shift that has been going on for 50 years and is only half complete.”

The article, written by John Parker: environment editor of The Economist, ends with these words: “For all of history, humans have lived in societies dominated (in numbers at least) by children. By 2060 children will be barely more numerous than any other age group up to 65. And looking after parents and grandparents will be as big as, or a bigger, social requirement ­as bringing up children and grandchildren. The year 2015 is, roughly, the halfway point in this astounding transformation.”

Further info:

Sri Lanka Department of Census and Statistics: 2012 Census Findings:

Census of Population and Housing 2012: Key Findings:

Interactive population pyramids for Sri Lanka (1950 – 2100):


Nalaka Gunawardene