Great question! I think there are two ways to answer this — one is whether we can predict completed cohort fertility rates in advance, and the other is whether we should rely on other metrics instead.
On predicting cohort fertility rates:
To some degree, I think we can. One way is with tempo-adjusted fertility rates, where you look at the average age of mothers giving birth in a given year and how that has changed over time, then adjust the total fertility rates accordingly. I’m slightly skeptical of this though, because the average age isn’t always that informative: you could see important changes at the tails of the age distribution, which affect the final rate in ways that an average wouldn’t capture. (There are also variations on this metric, of course.)
Another approach is to use cohort fertility rates up to a certain age (say, up to age 40) and then, based on assumptions about childbearing decisions and success rates after that, estimate what the final rate will be by the end of childbearing years. But this gets harder to project accurately, especially as new reproductive technologies make births more common at older ages and shift patterns in ways we haven’t seen before. Sometimes it’s not a monotonic delay—after the baby boom, for example, the US actually saw a widening of ages at childbirth, after the baby boom contracted that period.
These would be helpful if you’re interested in whether younger people are having more or fewer children over their lifetimes than older generations did.
But another way to look at it is, what question are people actually asking?
If someone just wants to know how many births happened in a year, they don’t care about how many will happen in the future, then looking at the actual number of births makes sense. This is useful for understanding the shape of the population pyramid, for example.
I’ve noticed that birth rates often trip people up. They’ve declined a lot over the 20th century partly because they use the whole population as the denominator — so as a country’s population ages, you’d expect the birth rate to fall, even if young adults are having just as many children as before. That’s why the total fertility rate (TFR) is generally more useful for this purpose: they use the population of women of childbearing age as the denominator, not the total population.
But if you’re specifically interested in the ratio of births to the whole population, then the crude birth rate is actually the right statistic to use.
Since you mentioned issues about the workforce, that often involves other factors also coming into play, like the size and productivity of the working-age population and how that’s changing. I’d be asking questions like Are women spending more time working? As people are living longer and healthier, are fewer people retiring early due to disability? Are people working more productively? How many dependants does each person actually have? As infectious diseases & mortality rates have fallen, are people spending less time out of work caring for their sick children, parents or relatives? etc. So I think looking at births alone could give a misleading picture of the demographic challenges that countries might face in the future.
Great question! I think there are two ways to answer this — one is whether we can predict completed cohort fertility rates in advance, and the other is whether we should rely on other metrics instead.
On predicting cohort fertility rates:
To some degree, I think we can. One way is with tempo-adjusted fertility rates, where you look at the average age of mothers giving birth in a given year and how that has changed over time, then adjust the total fertility rates accordingly. I’m slightly skeptical of this though, because the average age isn’t always that informative: you could see important changes at the tails of the age distribution, which affect the final rate in ways that an average wouldn’t capture. (There are also variations on this metric, of course.)
Another approach is to use cohort fertility rates up to a certain age (say, up to age 40) and then, based on assumptions about childbearing decisions and success rates after that, estimate what the final rate will be by the end of childbearing years. But this gets harder to project accurately, especially as new reproductive technologies make births more common at older ages and shift patterns in ways we haven’t seen before. Sometimes it’s not a monotonic delay—after the baby boom, for example, the US actually saw a widening of ages at childbirth, after the baby boom contracted that period.
These would be helpful if you’re interested in whether younger people are having more or fewer children over their lifetimes than older generations did.
But another way to look at it is, what question are people actually asking?
If someone just wants to know how many births happened in a year, they don’t care about how many will happen in the future, then looking at the actual number of births makes sense. This is useful for understanding the shape of the population pyramid, for example.
I’ve noticed that birth rates often trip people up. They’ve declined a lot over the 20th century partly because they use the whole population as the denominator — so as a country’s population ages, you’d expect the birth rate to fall, even if young adults are having just as many children as before. That’s why the total fertility rate (TFR) is generally more useful for this purpose: they use the population of women of childbearing age as the denominator, not the total population.
But if you’re specifically interested in the ratio of births to the whole population, then the crude birth rate is actually the right statistic to use.
Since you mentioned issues about the workforce, that often involves other factors also coming into play, like the size and productivity of the working-age population and how that’s changing. I’d be asking questions like Are women spending more time working? As people are living longer and healthier, are fewer people retiring early due to disability? Are people working more productively? How many dependants does each person actually have? As infectious diseases & mortality rates have fallen, are people spending less time out of work caring for their sick children, parents or relatives? etc. So I think looking at births alone could give a misleading picture of the demographic challenges that countries might face in the future.