Marion Burkimsher Affiliated to the University of Lausanne Spacing between children and trends in mean age of successive birth orders: quite different stories!

Assertion Spacing≠ difference in mean age of successive birth orders (Age at birth child x+1 – Age at birth child x ) ≠ (MABx+1– MABx) This is counter-intuitive! Spacing: relates to behaviour of individuals MABx: relates to mean of whole group

1.Model of spacing and Δ(MABx+1-MABx) 2.Spacing between children – empirical results 3.Mean age at birth of successive birth orders - data 4.Does the model have any relation to reality? 5.Relevance to real life

1.Model of spacing and Δ(MABx+1-MABx) 2.Spacing between children – empirical results 3.Mean age at birth of successive birth orders - data 4.Does the model have any relation to reality? 5.Relevance to real life

Mean age at 1st birth: 25 Std dev 0 Mean age at 2nd birth: 28 Std dev 0 Mean age at 3rd birth: 31 Std dev 0 Mean age at 4th birth: 34 Mean spacing between all birth orders: 3 years Mean age at last birth: 29.5 Age Scenario 1: 4 women, having 1, 2, 3 and 4 children respectively (MAB4-MAB3) = (MAB3-MAB2) = (MAB2-MAB1) = 3yrs

Mean age at 1st birth: 31.5 Std dev 3.9 Mean age at 2nd birth: 33 Std dev 3.0 Mean age at 3rd birth: 34.5 Std dev 2.1 Mean age at 4th birth: 36 Mean spacing between all birth orders: 3 years Mean age at last birth: 36 Age Scenario 2: 4 women, having 1, 2, 3 and 4 children respectively (MAB4-MAB3) = (MAB3-MAB2) = (MAB2-MAB1) = 1.5yrs

1.Model of spacing and Δ(MABx+1-MABx) 2.Spacing between children – empirical results 3.Mean age at birth of successive birth orders - data 4.Does the model have any relation to reality? 5.Relevance to real life

What is birth spacing? The mean gap between births of successive children It is, by definition, a cohort measure It is a very skewed distribution: mean>median>mode In Switzerland the mode is almost always a 2 year gap The likelihood of progressing on to a further child has changed much more than the interval between births

Probability distribution of 2 nd child after 1 st by period when 1 st child born Life table analysis, Swiss census 2000

Probability distribution of 3 rd child after 2 nd by period when 2 nd child born Life table analysis, Swiss census 2000

Probability distribution of 4 th child after 3 rd by period when 3 rd child born Life table analysis, Swiss census 2000

Note: * the median spacing associated with total number of children can only be calculated for those who have 2+ children, so is not exactly equivalent to other measures which include those who have only one Median duration between 1 st and 2 nd birth by these factors Life table analysis, Swiss census 2000

Summary on spacing Very small variations in birth spacing over time No trend towards wider or narrower birth spacing

1.Model of spacing and Δ(MABx+1-MABx) 2.Spacing between children – empirical results 3.Mean age at birth of successive birth orders - data 4.Does the model have any relation to reality? 5.Relevance to real life

Mean age of mother at 1 st, 2 nd, 3 rd, 4 th birth This can be calculated from birth registration records: period measure Since ~1998 biological birth order has been recorded; for before then, biological birth order distributions have been modelled (Burkimsher 2001) Can calculate the difference between mean age at 1 st birth and mean age at 2 nd birth, etc. (MAB2-MAB1….) Burkimsher, M Modelling biological birth order and comparison with census parity data in Switzerland: a report to complement the Swiss data in the Human Fertility Collection (HFC). MPIDR Technical Report TR (2011).

Mean age at 1 st, 2 nd, 3 rd and 4 th births Swiss birth registration (BEVNAT) Increase in MAB1: 5.3 years Increase in MAB2: 4.3 years Increase in MAB3: 2.9 years Increase in MAB4: 2.0 years (MAB2-MAB1) (MAB3-MAB2) (MAB4-MAB3)

Difference in mean age of successive birth orders Swiss birth registration (BEVNAT)

Standard deviation in mean age at 1 st, 2 nd, 3 rd, 4 th births Swiss birth registration (BEVNAT) Increases Std dev MAB1: 0.8 Std dev MAB2: 0.5 Std dev MAB3: 0.2 Std dev MAB4: 0

Summary of empirical observations From 1950s-1990s (using census 2000 data): Spacing between all births has stayed ~constant Median spacing between births ~3 yrs (mode=2yrs) From 1969 to 2013 (using birth registration data) (MAB2-MAB1) declined from 3yrs to 2 yrs (MAB3-MAB2) declined from 2.8 to 1.2 yrs (MAB4-MAB3) declined from 2.4 to 1.2 yrs

Why the difference between spacing and ΔMABx? Because they relate to different subsets of women! Women who start their family younger in life have a greater likelihood of having a larger family, and v.v. Why the trends in ΔMABx post-1969? ‘All’ women used to have their 1 st child around the same age: since 1970 there has been a divergence of behaviour Transition from Scenario 1 to Scenario 2

1.Model of spacing and Δ(MABx+1-MABx) 2.Spacing between children – empirical results 3.Mean age at birth of successive birth orders - data 4.Does the model have any relation to reality? 5.Relevance to real life

Mean age at 1st birth: 25 Std dev 0 Mean age at 2nd birth: 28 Std dev 0 Mean age at 3rd birth: 31 Std dev 0 Mean age at 4th birth: 34 Mean spacing between all birth orders: 3 years Mean age at last birth: 29.5 Age Scenario 1: 4 women, having 1, 2, 3 and 4 children respectively (MAB4-MAB3) = (MAB3-MAB2) = (MAB2-MAB1) = 3yrs ~1969

Mean age at 1st birth: 31.5 Std dev 3.9 Mean age at 2nd birth: 33 Std dev 3.0 Mean age at 3rd birth: 34.5 Std dev 2.1 Mean age at 4th birth: 36 Mean spacing between all birth orders: 3 years Mean age at last birth: 36 Age Scenario 2: 4 women, having 1, 2, 3 and 4 children respectively (MAB4-MAB3) = (MAB3-MAB2) = (MAB2-MAB1) = 1.5yrs ~2008

How well does the model match reality? Changes in MABx Changes in (MABx+1 – MABx) Changes in Std dev MABx

Δ yr 1-yr2ModelActual MAB125->31.5 = +6.5yrs25.1->30.4 = +5.3yrs MAB228->33 = +5yrs28.0->32.3 = +4.3yrs MAB331->34.5 = +3.5yrs30.7->33.6 = +2.9yrs MAB434->36 = +2yrs32.8->34.8 = +2.0yrs

Δ yr 1-yr2ModelActual (MAB2 – MAB1)3->1.5 = -1.5yrs3->2 = -1yr (MAB3 – MAB2)3->1.5 = -1.5yrs2.8->1.2 = -1.6yrs (MAB4 – MAB3)3->1.5 = -1.5yrs2.4->1.2 = -1.2yrs

Δ yr 1-yr2ModelActual Std dev MAB10->3.9 = +3.9yrs4.5->5.3 = +0.8yrs Std dev MAB20->3.0 = +3.0yrs4.2->4.7 = +0.5yrs Std dev MAB30->2.1 = +2.1yrs4.2->4.4 = +0.2yrs Std dev MAB4-4.2->4.2 = 0yrs

1.Model of spacing and Δ(MABx+1-MABx) 2.Spacing between children – empirical results 3.Mean age at birth of successive birth orders - data 4.Does the model have any relation to reality? 5.Relevance to real life

Why is this relevant? Do not make incorrect deductions as in Houle & Shkolnikov (2006)* Spacing data is essential for forecasting births and predicting evolution of TFR, using method proposed by McDonald and Kippen (2011)* Change in fertility behaviour has implications for type of contraceptive requirements Temporary, reversible contraception Possible permanent contraception

Conclusion A complicated, little studied aspect of fertility studies… Feedback welcome! *Houle, René and Vladimir M. Shkolnikov, Regularities and peculiarities of birth schedules in industrialized countries: an analysis of FFS data. MPIDR Working Paper WP Rostock. *McDonald, Peter and Rebecca Kippen Forecasting Births, Feature Article, Cat. no , Australian Bureau of Statistics, Canberra

Thank you!

Differences in MABx by education Highly educated women start their families later, but differentials by family size similar

Mean age at 1 st birth by educational level Swiss census years 5.3 years

Variation in timing of 1 st birth by educational level Swiss census 2000

Differences in MABx (tertiary – low educated) Swiss census 2000