If at the end of last year you felt there was a bit more room for you to stretch out your arms, you were right. For the first time since 1916, when thousands of young men were heading overseas to fight in the first world war, Australia’s population shrank.

The bureau of statistics estimates that in September last year there were 4,239 fewer residents in this country than there had been in June.

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That may not seem like an astounding drop, but normally our nation’s population increases by about 100,000 every three months.

Admittedly only New South Wales, Victoria and the Australian Capital Territory actually saw a fall in population, but all other states experienced a sharp slowing in the growth of residents.

The big reason was border closures. Net migration, which normally increases by about 60,000 every three months, fell by 34,804. The bulk of this was in Victoria where 18,438 more people left the state to go overseas than arrived from another country.

Australian residents were also not moving to Victoria – obviously because of border restrictions. In the six months from April to September last year, 7,691 more people moved out of the state to go somewhere else than moved to it. It was the first time since 2007 that for two quarters in a row more people left than arrived.

But over in South Australia for the first time since 1991 in two consecutive quarters more people moved to the state than left it.

It’s not just migration however, that is the cause of our falling population.

There were fewer births per head of our population in the September quarter last year than at any time on record.

There were just 30,565 births in July, August and September last year – equivalent to 119 per 100,000 people.

Our falling birthrate, however, has been a long-term trend.

You don’t generally plan to have children when the economy is not performing well, or when your standard of living is not increasing. As such it is not surprising that since the GFC, our birthrate has been steadily declining.

A decade ago there would be around 185 births per 100,000 people every three months – a level that would have seen an extra 15,000 or so babies born between July and September last year.

All of this affects the economy. Over the past decade, nearly two-thirds of growth in the economy comes from population increase. Remove that growth and our economy will necessarily be smaller than we otherwise would expect it to be.

There will be some cheering, suggesting that this will improve congestion and housing affordability, when in reality it will have little impact, and neither will it have a great impact on jobs.

For all the talk about migrants coming and taking our jobs, what we have seen over the past year is that overwhelmingly the fall in migration has been among those in their 20s.

The ABS estimates that the population of people aged 20-24 fell 3% last year, and those aged 25-29 went down 1.5%.

By contrast, the number of people aged over 65 rose 3.4%.

What happened? Foreign students left, and elderly Australians living overseas came home.

The first three months in 2020 saw a surge of people coming home, as they realised there were not much better places to sit out the pandemic than Australia.

And at the same time foreign residents – especially students – were leaving the country and those who would normally have been arriving to start study were not.

So if you go to a university you might definitely experience fewer people around; you will also experience lower funding for your courses as a result.

Once the vaccine rollout becomes widespread enough for travel to open, these migration numbers will return. But it will be interesting to see if the lockdown periods in the middle of last year do lead to an increase in birthrates, or if fears of the future will continue to see Australians more than ever before choosing not to increase the size of their families.