Chapter 1 Biological Perspective: Considering the Essence of the Declining Birthrate 1.2 Why Human Birthrates Decline in an Era of Low Mortality

1.2 Why Human Birthrates Decline in an Era of Low Mortality

"People in the past had more children, so why is the birthrate declining now?"

Whenever the declining birthrate is discussed, some people likely have this question. In the Japan of the past, large families were the norm, and it was common to have three or four children. No, in many places, there were even more. Incidentally, I myself am one of five siblings. However, what about now? "Only-child families" without siblings are not rare.

This change is not just a matter of social values. Could it be related to a more fundamental change in our "survival strategy" as biological organisms?

In this section, let's consider why birthrates fall in an era where death has decreased.

1. The past was an era of "having many children to survive"

In past societies, having many children was a form of "risk management."

(1) High infant mortality rate

It is said that in Edo-period Japan, nearly half of all children born died before the age of five. While unthinkable today, it was not easy for small children to survive due to infectious diseases, malnutrition, and accidents.

• Underdeveloped medicine: It was not rare to lose one's life even from pneumonia or diarrhea.

• Malnutrition: Mothers could not get enough nutrition, and babies were weak.

• Unsanitary environments: Well water or river water could sometimes cause illness.

In this situation, it was necessary to build a family under the premise that "not all children born will necessarily reach adulthood." That is precisely why it was normal to have many children.

(2) Population decline due to war and epidemics

Looking back at human history, we see a phenomenon where birthrates rise whenever there is a war or an outbreak of infectious disease.

• The Plague (Black Death): 30-50% of the population in 14th-century Europe was lost, but the birthrate surged afterward.

• Japan during the Sengoku period or after World War II: After many lives were lost in war, the population recovered rapidly.

The pattern of "population decreases -> we must quickly increase the next generation" may have been an instinctive adaptation as biological organisms.

2. Do the development of medicine and hygiene lower the birthrate?

Entering the modern era, medical and sanitary environments improved dramatically. Because of this, there is no longer a need to "have many children to increase the probability of survival" as in the past.

(1) An era where children almost always survive

• Spread of vaccines → Infectious diseases like smallpox, measles, and polio have been overcome.

• Advances in neonatal medicine → Survival rates for low-birth-weight and premature infants have improved significantly.

• Improvement in nutritional environments → Famine has decreased, and the probability of babies growing up healthy has increased.

Nowadays, it has become the norm that "if you have two children, both will reach adulthood." Consequently, the awareness that "you must have many children for the family line to continue" has disappeared, and it can be said that a decline in the birthrate is a natural progression.

(2) Increased longevity slows down the speed of generational turnover

• Average life expectancy in the past: Under 50 → "Because parents died early, it was necessary to have children early to ensure generational turnover."

• Average life expectancy today: Over 80 → "Because parents live longer, there is no longer a need to rush to have children."

If the parent generation can live long and healthy lives, the speed at which the next generation increases will naturally slow down.

(3) Rather than "having many," the "cost per child has increased"

In the past, "increasing numbers" was the top priority, but now there has been a shift toward "quality over quantity."

• Increase in educational expenses: We are in an era where "going to university is the norm," and the cost per child has increased.

• Changes in parental awareness: The value of "wanting to provide the best possible environment" has become stronger.

• Increase in dual-income households: Time available for child-rearing has decreased, and more families are choosing to limit the number of children.

Precisely because we live in an "era where people don't die," it has become an era of "reducing numbers and, in return, investing more time and cost into each individual."

3. Summary: The decrease in deaths and the decline in birthrates occur as a set

✅ In the past, the mortality rate was high, and the family line would not continue unless many children were born
✅ With the development of medicine and hygiene, society has changed to one where "there is no need to have many children"
✅ Due to increased longevity, the pace of generational turnover has slowed
✅ The declining birthrate can also be considered a result of humanity shifting to a "more sustainable reproductive strategy"

Rather than viewing the "declining birthrate as a problem," isn't it important to understand "why the declining birthrate is progressing?"

Next time preview!

Next, let's think about "1.3 Global Environment and Resource Constraints: Sustainability Brought About by the Declining Birthrate"!

Humanity has historically considered 'increasing the population' to be synonymous with 'progress.' However, the Earth's resources are finite.

'Is a declining birthrate actually a good thing for the Earth?' Let's delve deeper into this from a biological perspective next time!

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