In 1990, both countries' demographics resembled a pyramid which meant that there were far more young people being born than there were old people.

But fast forward to projections for 2050 and you'll notice a dramatic change.

The Chinese pyramid narrows to resemble a shape more like a funnel.

For more than two centuries, China has had the largest population in the world.

This year, the UN said India will take its place.

India's population is expected to keep growing for the next four decades, peaking at nearly 1.7 billion in 2063.

China's population is projected to shrink rapidly.

By the start of the next century, India's population is expected to be double that of China's.

All the science point to a very challenging future for China, demographically.

Here's why this shift marks a watershed moment for both nations and what the change could mean for the global economy.

This year, China announced its first population decline since 1961.

In 2020, China's fertility rate or the number of children a woman has over her lifetime came in at 1.3 which is one of the lowest in the world.

The roots of China's demographic crisis can be found in its one-child policy.

The policy was designed to keep birth low so that China would have the opportunity to grow its economy.

It turns out, in reality, the one-child policy has far more long-lasting impacts the policy makers intended.

In a way, it worked too well.

Starting in 1980, most Chinese couples were strictly limited to one child.

Despite reversing the one-child policy in 2016, China's fertility rate is continued to decline.

The replacement rate needed to keep a population stable is 2.1.

Meaning that a woman would need to give birth to at least two children to keep the population stable and essentially replace the parents.

With a fertility rate of 2.0, India's fertility rate is slightly lower than the replacement rate.

The reason why India's population is projected to keep growing for several more decades is because India, unlike China, still has a fairly large group of women of reproductive age that's very different from China.

In China, the one-child policy produced a dramatic gender imbalance that originated in strong cultural and social preferences for male children.

In 2021, the number of marriage registrations plummeted to 7.6 million, the lowest since the government started keeping records in the mid-1980s.

The global economy has grown to rely on China's vast pool of factory workers for manufactured goods and its consumers represent a growing market for western-made cars and luxury goods.

A declining Chinese population would mean that there will be fewer workers to make products that are exported to other countries and there will be fewer consumers,

especially younger consumers, to buy goods from other countries.

In the coming decades, demographers expect China will face a huge worker to retiree gap.

In 2020, 14% of China's population was over 64 years old.

While in India, just 7% of people were over 64.

By 2070, nearly 59% of people in China will be over the age of 64.

While in India, only 30% of the population will be in the same age bracket.

A larger group of retirees will mean that younger Chinese people will have a bigger of a burden to support the elderly.

That's especially true in the rural areas.

In some ways, India looks like China did 30 years ago.

It has a rapidly expanding working age population with 610 million people under age 25 and relatively few older people to care for.

India's rising population means it's likely to keep its economy growing, buy more of the world's goods and play a bigger role in global affairs.

But India's rise isn't assured.

India's problem now is all about jobs.

Yes, it does have a large working age population but at the same time it is struggling to create jobs for its young people.

India is primarily a rural nation and still lags behind China in urbanization.

Unlike China, where millions of migrant laborers move to cities to work in factories, many Indians are reluctant to leave their hometowns.

India also significantly lags behind China in terms of female labor participation.

India's cultural norms have meant that families who can't afford to keep their daughters and wives at home prefer to do so.

When Mao took power, he really pushed the idea that women is equal to men.

For a long time, China's women labor participation rate has been very high and for India, that seemed to be not the case.

Whatever happens, the world's future population will be tilted more towards the Global South and South Asia, in particular.

In 2060, the three successor states to pre-independence India will have 2.3 billion people versus China's 1.2 billion guaranteeing profound changes to the global economy.