My earlier post on birth rates generated some interest and discussion and therefore deserves a bit more attention and clarification. First, the average birth rate referenced in news headlines is simply the number of births divided by the number of women age 15-44. This average birth rate conflates age-specific changes in birth rates with changes in the age distribution of the population. The average birth rate is lower in 2011 in part because there are so many women age 15-19 and age 40-44, when birth rates are low.
Demographers and labor economists use the total fertility rate to study changes in birth rates over time. The total fertility rate gives equal weight to women of each age from 15 to 44 and therefore is invariant to changes in the age distribution of the population. My second point is that the total fertility rate in 2011 is higher than it was from 1975 to 1985 and therefore is not the “lowest ever.” Finally, the total fertility rate understates the expected number of children born to women from a given birth cohort when women are choosing to delay childbirth. Delayed childbirth will show up as an initial decline in the total fertility rate but not the number of children per woman.
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.
I do not believe that the delay in family formation we have seen thus far is due to decadence. Couples are likely delaying children because of the weak economy and the high costs of raising children. Columnists and commentators should write and talk about the dramatic changes that will occur in our society when the majority of first-time parents are in their thirties. I am skeptical, however, that one of the changes will be a decline in the U.S. population.