Recently, the economist Stephen Bronars wrote a post suggesting that while U.S. women are delaying childbearing, this does not necessarily entail a decline in fertility. In a follow-up post, he notes the following:
By following a given birth cohort of women from the ages of 15 to 44 I calculate that:
* Women born between 1956 and 1960 had an average of 2.03 children
* Women born between 1961 and 1965 had an average of 2.06 children
* Women born between 1966 and 1970 had an average of 2.12 children
* Women born between 1971 and 1975 had an average of 2.11 children
The youngest of the women born between 1971 and 1975 are age 37 and are likely to have more children. Thus 2.11 is an underestimate of the average number of children born per woman for the 1971-1975 birth cohort. There is no evidence of declining birth rates among women who are at or near the end of their childbearing years. For women born more recently (1976-1980 or 1981-1985) there are still many more years of possible childbearing.
More recent cohorts of women are having fewer and fewer children before the age of 25. They are also likely to have more children in their 30’s than their older sisters, mothers and grandmothers. It is far too early to predict whether lower birth rates for women under the age of 25 in 2011 will translate to lower birth rates over their lifetimes. What does seem clear is that we are transitioning to a society where the 30’s become the prime childbearing ages for most women. It is already true that there are more births (per capita) to women age 30-34 than age 20-24. [Emphasis added]
As a friend kindly explained it to me, one of the reasons we might intuitively associate delayed childbearing with declining fertility is that if we lived in a society in which all women had one child per year starting at age 16 and kept going for as long as they possibly could, moving the start date to 25 or 35 would necessarily lead to declining fertility. But in societies in which women have more agency over fertility, things don’t work that way. Rather, women could plausibly reach modest fertility targets — in the neighborhood of two children, for example — whether they start at 25 or 35.
But there is a problem, which is that fertility tends to decline with age. Women who start at 35 might have a somewhat more difficult time getting pregnant again at 39 or 42, etc., than women who start at 25 and have a second child around age 30. Moreover, a woman who starts earlier in life and has two children in her 20s might decide to have a third child in her 30s as her life circumstances change. A woman who starts in her late 30s is on average far less likely to be able to give birth ten years on.
Bronars is right that we might see some sharp increase in fertility in today’s cohort of under-30 women as they age and as broader economic conditions improve. But even as assisted reproduction grows more advanced, there will remain limits on the ability on the fertility of women in their late 30s and 40s. And if current trends continue, the cost of childrearing in terms of foregone wages and human capital outlays is likely to keep increasing.