Nickerson and Solomon include fixed effects to control for the overall fertility in a county and year, allowing it to vary by the number of children a woman has. This variation is measured at the state-year level using the 2000-2004 ACS surveys, which controls for variation in overall birthrates due to many economic and demographic factors, and allows for changes in overall desired family size in that county and year. This is important for distinguishing the effect of car seat laws from the impact of other secular changes that might affect birth rates.

Overall Fertility

Nickerson and Solomon include dummy variables for every combination of children’s ages. In other words, they also estimate a general probability of giving birth for each combination of children’s ages, such that the effect of car seat laws is measured relative to other families with the same number and ages of children. This allows them to measure the effect of car seat laws relative to the overall patterns in the spacing of children’s births, while putting very few structural assumptions on the shape of such birth patterns.

Combinations of Children's Ages

- Yearly values of a woman's age
- Race/ethnicity
- Quintiles of household income
- Highest education level among household members
- Presence of a male in the household

Demographic Variables

To distinguish the effect of car seat laws from other factors that will affect birth rates, Nickerson and Solomon employ numerous controls, fixed effects and interaction terms in their specifications.

University of Minnesota - Twin Cities

Nickerson and Solomon use a panel regression setting where the main dependent variable is a dummy variable equal to one if the woman in question gave birth that year. Their main independent variable of interest is Two Children, Both Bound. This takes a value of one if the woman already has exactly two children whose age and state-year mandate requires they use child safety seats. 

Methodology

Nickerson, Jordan and Solomon, David H., Car Seats as Contraception (July 31, 2020). Available at SSRN: https://ssrn.com/abstract=3665046 or http://dx.doi.org/10.2139/ssrn.3665046

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