If back at the end of 2019 someone was to tell you that just over two years later we would be debating when will the unemployment rate go below 4%, you would have been excused for thinking that the number of employed must have surged a lot faster than it actually has.

Prior to 2020 employment was growing around 4.2% every two years, so you would have been excused from thinking to get unemployment down so low, job growth must have taken off and we would be well above 13.5 million people employed.

And yet, no. Employment since the end of 2019 has increased just 2.1% – in effect at half the speed it had been in the period up to the pandemic:

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The level of employment is around 2.2% below what would have been expected, but the key thing to remember is that the unemployment rate is a percentage (the ABC’s Gareth Hutchens has some funky videos explaining this).

It is related to the number of people in the labour force – and one of the big changes since the pandemic is the absence of migration has meant the pool of working age people has barely grown at all:

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And thus with a barely growing labour force, employment does not need to grow as fast in order to see unemployment fall.

As it is, the level of unemployment is well below what would have been expected a couple of years ago:

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This does not mean it is irrelevant that unemployment is low, just that it is always worth remembering the current labour market is weird and in some ways artificial – not just due to the limit on migration but also due to the massive level of fiscal and monetary stimulus that is pumping up demand.

The last time the unemployment rate fell to 4.0% in 2008 we were still benefiting from the mining boom, the GFC had yet to arrive, the budget was in surplus and the Reserve Bank of Australia had increased interest rates seven times in three years to take the heat out of the economy:

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We are not in such a situation now, and neither are we in a situation as was the case when the unemployment last had a 3 in front of it – back in the November quarter of 1974.

It is quite remarkable how greatly our labour force has changed since then – a time so far removed that the ABS didn’t even do monthly unemployment figures!

The first big change is that our labour force is older now. In 1974, 45% of our working-age population was under 35; now it is just 33%:

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But the biggest difference is the gender makeup. And it explains somewhat why unemployment was so low then – ironically it was for an artificial reason as is currently the case.

While our current labour force is restricted by migration limits, back in 1974 it was restricted by both laws and conventions that kept women out of work.

In 1974 just 35% of those in the labour force were women; now it is 47%:

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The difference can be seen in the breakdown of the age of those working.

In 1974 the most likely age a woman would work was in their early 20s. Then the “nappy valley” would occur: women leaving the labour force, having kids and essentially never returning.

Men on the other hand did not seriously leave the labour force until their early 60s:

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Now things are much more equitable (if not completely so).

The current most likely age for women to work is their 40s and 50s and the nappy valley has become a flat land.

Women don’t have the steady increase that men do in their late 20s and early 30s, but there is no longer the large exit from work:

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Another key change is that women are now more likely to work full-time than in the past. In 1974 just 30% of women in their late 20s worked full-time, now half do:

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Here we still see evidence of women leaving to have children – although it now occurs in their 30s rather than 20s and it is more likely to involve going from full-time to part-time.

For men, however, the story is the quite different. Men are now less likely to work full-time than their counterparts 48 years ago – due to the shift towards more “flexible work arrangements”:

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But despite these movements, the gender gap of full-time work remains very much in force, with men in their 30s and 40s being almost twice more likely to work full-time than women at that age:

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And so the unemployment rate is likely to get below 4% sometime this year – it’s already at 4.2%. But it is worth reflecting that we got to that level due to very unusual circumstances, and that just looking at the rate itself hides a lot of what is going on in the labour market.

We have a very different economy to what we had the last time the unemployment rate was under 4%, but what unites both now and 1974 is that a reduced labour force might provide a better looking unemployment rate, but not necessarily a better economy or community.

• Greg Jericho is a Guardian columnist and policy director at the Centre for Future Work