New Way to Look at Population Control

I was trying to figure out how many people have lived since the beginning of time (and how many of them are currently alive), which made me think about population models with birth rates and death rates. Which made me think about the mythical 2.1 children we are told keeps the population steady. Then it dawned on me that this wasn't really true. Or rather it wasn't the whole truth. The age at which mothers have their children impacts the population as well. The younger a mother is when she has her children, the larger the impact on the birth rate (and population).

But, you never hear people talking about having mothers wait to have kids as a way of lowering the birth rate. For example this report on population trends in India doesn't mention the average age of mothers at birth in it at all. All they mention is the Total Fertility Rate (TFR). They seem to think a baby is a baby regardless of how old the mother is. And other population control literature I looked at also just looks at how many kids each mother has.

The way I thought about this was if you have a population of 8 people (really small world) who all have 2 kids at age 20 and die at age 80, then you have 4 generation living at any one time: 2 1 yr olds, 2 21 yr olds, 2 41 yr olds, 2 61 yr olds. If they waited until 40 to have kids their would only be 4 people (2 generations) living at one time. Likewise if they could have kids at age 10 there would be 16 people (8 generations).

Interestingly the math becomes very similar to that of comparing the gas consumption of cars. You can think of the mother who has kids as an early age as the Hummers and a mother who has kids at an later age as a Prius. Instead of gallons/mile we are talking kids/age of mother. A woman who has 1 baby at 15 yrs old has the same impact (1 per 15 yrs or 1/15) on the birth rate as a woman who has 2 at 30 (2/30 or 1 per 15 yrs) or 3 at 45. A woman who has 2 kids at 15 and 17, has the same impact on the birth rate as a mother who has 3 kids at 21, 25 and 26 (1/15+1/17=1/21+1/25+1/26).

Wouldn't it be easier for China to control population if instead of having a 1 child policy, they had a 1 child before the age of 18 or 2 after the age of 35 policy? Wouldn't it be easier to convince mothers to delay having kids than to not have them at all?

Waiting from 15 to 20 is decreases the impact of that birth by 25%. This is the same impact that decreasing a family from 4 kids to 3 kids has (assuming that they were all born on the same year). And like the automobiles, the people who have kids at the earliest age (the Hummers) have the biggest impact on birth rates. Getting 15 yr olds to wait 5 more years has a bigger impact then getting 25 yr olds to wait 5 yrs (only 16%).

In developed nations it is happening as well. Based on this report, in the US the average (or mean) age of mothers for all births rose from 24.6 years to 27.2 over the past three decades. This decreases the birth rate 10%.

This report shows South Korea from has changed the average age for people having their firstborn from 24.9 in 1985 to 28.3 in 2002 or a decrease of 12%.

So there you have it. To control population growth, focus on delaying having kids as well as reducing the total number of kids. And forget about that 2.1 number.