The birth rate has declined but the population is still growing. It takes decades for a decline in birth to be felt across the entire population size, as most of the people who were born before the decline are still around.

The birth rate is declining, but people are living longer. It will take a generation for any change to be noticed.

A " birthrate has declined by 20%" do not mean there is not an increase in population, if it was high enough to begin with you can still have an increase it is just slower.

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Even if you have a birth rate that over a long time would result in a stable population it can still increase. Let say it would end with 10 000 births and 10 000 deaths per year. The population is 10 000 x average lifespan, so if the life expectancy increases the population can grow even if the birth rate is constant.

The average human lifespan on earth during the 20th century increased from 48 to 74 years for men, and from 51 to almost 80 years for women. that is close to 35% increase for both. That is a major part of the population increase on the earth

But there is another major factor for population density other than birth rate is migration both inside and between countries.

So even if the birth rate in your country would result in minimal growth migration from other countries can increase the population.

The general trend today is people moving to large cities from the countryside and smaller cities. So if you live in a large city you will see a population increase that primarily is a result of people moving there. The change is not primarily because if birth but migration.

If you were out in the countryside you would likely see a population decrease. It is a result of people moving to cities. Because young people are more likely to move than old the demographics can become quite uneven and as a result, the number of deaths can be a lot higher than the number of births.

If you have population of 1000 with birthrate of 20 per 1000. You get 20 kids.

If you have population of 2000 with birthrate of 16 per 1000. You get 32 kids.

This is just an example, but effectively birthrate can go down while at the same time absolute number of births goes up.

At the same time there is a global trend that populations are concentrating in Urban areas.

For example in the US 80% of the population lives in Urban areas and US Census Bureau only considers 3% of the country's land area Urban.

Also depending on which country you are from immigration can compensate for falling birthrate.

Because we forgot what life was like when the birth rate was higher.

My dad slept on a cot in his parents room until his oldest brother went off to college. After that he shared a room with two bunk beds with three of his other brothers.

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Plus if you live in city it feels much worse than it actually is. Take a drive through the dakotas and you’ll see just how much of nothing there is

A) I do t know the relation to the death rate, but before the decline, the birth rate was very clearly higher than the death rate. If the 20% decline caused it to equal or drop below the death rate, then see B). But if the birth rate got closer to the death rate, yet is still higher, then population will continue to increase. However, it will increase at a slower rate.

B) any decrease in population is unlikely to be felt until 18-25 years later. Until then, those children are still mostly just following their parents around or perhaps are in college which is almost like its own little contained ecosystem. You are unlikely to feel like they are making things more crowded. During that 10-25 years, there are children born during the faster birth rate time who are becoming adults and entering the world, making it seem more crowded.

C) I would be very doubtful of any study that asks people if it "seems more crowded" right now for several reasons. Mostly, we just got out of a global pandemic that held our country in grip for nearly 2 years. Maybe you weren't isolating, but a huge percent of the population were. So you are definitely experiencing a surge in people going out in public. You may think it seems more crowded than before the pandemic, but it's so hard to overcome implicit bias in the way we perceive the world. That's why rigorously controlled studies are required. We need to know how many people trafficked a certain area in. A certain time 3 years ago, and we need to average over several days of the week because their schedules and hobbies and reasons for going outside are not only dependent on that, but may have changed in the last 3 years. We should probably even consider time of year. We went into the pandemic during winter, and really this spring and summer was when the majority of people I knew started going back to life as it was before (more or less). So the study should either average across multiple times throughout the year, or track month, temperature, weather, and all factors to make sure they aren't affecting the number of people that are out and about. Your brain does not do any of this automatically. I am highly suspicious that your anecdotal evidence actually implies things are more crowded. In fact, I even sorta agree with you, I never seemed to have to wait for the self-checkout at Walmart but maybe 2 or 3 times a year at the worst. Now it seems like I'm waiting more often than not. I thought that was weird until I realized I also just moved from a city of 75k to over 400k. Maybe waiting is more common here. Maybe people are less likely to choose the person operated checkout because they're so use to isolating, they're not used to interacting. Maybe Walmart has fewer people on duty to checkout either due to the pandemic itself or to keep pace with consumers, but they haven't yet bounced back fully. There are so many variables.

Assuming this statement is true (do you have a source) there’s multiple things going on here.

1. A decline in the birth rate does not necessarily mean a decreasing population.

First population change is given by:

Total population - deaths + births.

As long as births > deaths your population will grow.

For example, let’s say in 2020 you had a population of 1,000, a death rate of 10% and a birth rate of 20%. That would give you:

1,000 - 100 + 200 = 1,100 people. A growing population.

Let’s look at the next year. For simplicity sake 100 people moved away so our population is back to 1,000. Death rate stays the same, 10%, but birth rate declined from 20% to 16% (a 20% decrease).

Your population equation is:

1,000 - 100 + 160 = 1,060.

Not growing as fast but still growing.

2. Even if you had an overall declining population, that doesn’t mean the decline is the same everywhere. Some areas could be declining more quickly while others are declining more slowly or even increasing.

3. People move. Even an overall declining population doesn’t mean a particular area will become less crowded if more people are moving there.

4. It takes time for changes in population growth rates to be felt. Especially if there isn’t a consistent change over time. If that 20% decline is replaced in a few years by 20% increase you’ll see things even out over time. Aside from your kids class being more or less crowded for a few years you might not notice the change at all.

Say you have a population of 50 people that all live in the same town and all of them are in M/F couples (so 26 couples) and over a year every single one of them have a baby, so 26 babies.

Now you have a total town population of 76.

Now lets take that exact same town but there's 100 people living in it. Same deal, all of them are in couples, but this time only half of the couples have children (so 50 couples, 25 children) now there's a total population of 125 in the town.

The first town has a much higher birth rate, but the second town is more crowded.