For those wondering, TFR = total fertility rate and means "total number of children born or likely to be born to a woman in her life time if she were subject to the prevailing rate of age-specific fertility in the population". [1]

Essentially, how many children a woman will give birth to, on average. If it drops below 2, population will begin to shrink.

Edit: As the below comment thread points out, > 2 (~2.1) may be required to not drop. :)

[1] http://www.searo.who.int/entity/health_situation_trends/data...

Not to be pedantic but I think it needs to be a higher than 2 in order to not shrink. The number usually stated is 2.1. This is to account for people who die before having reproduced themselves.

Pretty sure that it already accounts for that. As far as I understand it is for the entire female population.

Say you have 100 females.

25 have no kids 15 have 1 kid 25 have 2 kids each 15 have 3 kids 10 have 4 kids 10 have 5 kids

This would be a 2.0 TFR and would lead to no shrinkage or increase.

The 25 without kids would include those who die before reproducing.

Now a potential reason it might need to be higher than 2.0 is if it is not a 50/50 split between male and female children. I vaguely recall that slightly more male children are born than girls but it averages out by adulthood because boys are slightly more likely to die doing something stupid. This may not actually be the case so do some research before quoting me on the more boys part.

The academics/experts in the field generally consider 2.1 to be replacement level fertility because:

Accidental deaths before having kids

Gay/Lesbian kids

Kids that “want” to have kids but for whatever reason are unable to

But surely people who don't have kids are already part of calculating what the fertility rate is? Why would you manually exclude parts of the population, only to add a skew factor later to your number to effectively reinclude them?

I totally believe you that 2.1 is the standard, but I don't understand why based on the explanations in this thread.

Shouldn't "people who don't have kids" be accounted for by lowering the overall measure of "children a woman will give birth to on average"? So if half of women in a country have exactly four kids, and the other half have zero kids for whatever reason, the average number per woman is 2.

The reason is that TFR is not the average number of children per woman over her lifetime.

Instead, it is calculated by measuring the average number of children born to women at a specific age (e.g. 15, 16..49). Adding all of these up results in the TFR. Thus mortality rates are not included in the TFR.

> Kids that “want” to have kids but for whatever reason are unable to

The opposite is also likely. People may have "accidental" kids, or they could change their mind.

(I still agree that 2.1 is a good approximation, but it is also likely to vary on cultural and natural effects)

Per wikipedia, what you are describing is the Net Reproduction Rate.

TFR is not calculated that way. Instead, it takes women in the child bearing age group (say 15-44 or 15-49), and adds up the age specific fertility rates to calculate the TFR.

What TFR would result in replacement level of the population depends to some extent on mortality rates among women from birth to end of reproductive age, Thus advanced countries may achieve replacement levels at a TFR of 2.05 (say), while countries like India may do so at 2.1, and other countries with higher mortality may do so at 2.3+.

...and because more males are born than females. (105:100)

I guess this makes the assumption that each woman is impregnated by different men. If 10 women and one man had a TFR of 1.5 the population is still growing no?

If you mean: some weird tribe that has a population of 10 grown up women and one grown up men then yes, any TFR > 1.1 would mean population growth.

But in normal circumstances you assume that there is an equal number of men and women in population. Or, to be more precise, you assume 105:100 men:women ratio, because that's what statistics show in most countries. That's why you need TFR slighly higher than 2 to have constant population size.

Populations usually have close to 50% split, although I think there are usually slightly more woman.

Outside of war/post war its unlikely to have a situation where there are 10 woman to each man.

India has a history of favoring males, so many females are aborted. You are more likely to see 10 men to each woman.

>You are more likely to see 10 men to each woman

So you are saying India's new-born gender divide is 91% men and 9% women?

Abortion based on gender is illegal in India. Most (all?) gynecologists in India won't reveal gender due to this until a certain stage (20 weeks) in the pregnancy after which abortion is illegal (requires court order).

Illegal it may be, but there are many states in India where the sex ratio for babies is badly out of whack.

https://scroll.in/pulse/834701/one-step-forward-two-back-ind...