Thursday, May 9, 2019

Less problems for baby in spring birth causes an average marginal willingness to pay for a spring birth to be $877; this implies a willingness to trade‐off 560 grams of birth weight to get a spring birth

The Demand for Season of Birth. Damian Clarke, Sonia Oreffice. Climent Quintana‐Domeque. Journal of Applied Econometrics, May 2 2019.

Summary: We study the determinants of season of birth for married women aged 20‐45 in the US, using birth certificate and Census data. We also elicit the willingness to pay for season of birth through discrete choice experiments implemented on the Amazon Mechanical Turk platform. We document that the probability of a spring first birth is significantly related to mother's age, education, race, ethnicity, smoking status during pregnancy, receiving WIC food benefits during pregnancy, pre‐pregnancy obesity and the mother working in “education, training, and library” occupations, whereas among unmarried women without a father acknowledged on their child's birth certificate, all our findings are muted. A summer first birth does not depend on socioeconomic characteristics, although it is the most common birth season in the US. Among married women aged 20‐45, we estimate the average marginal willingness to pay (WTP) for a spring birth to be 877 USD. This implies a willingness to trade‐off 560 grams of birth weight to achieve a spring birth. Finally, we estimate that an increase of 1,000 USD in the predicted marginal WTP for a spring birth is associated with a 15 pp increase in the probability of obtaining an actual spring birth.

Keywords: quarter of birth, fertility timing, willingness to pay, discrete choice experiments.

1 Introduction

While the relevance of season of birth has been acknowledged at least since Huntington’s 1938 book “Season of Birth: Its Relation to Human Abilities”, it was not until recently that season of birth became prominent in biology, economics and social sciences more generally.There is now a well-established literature illustrating a variety of aspects that are significantly correlated with season of birth, including birth weight, education, earnings, height, life expectancy, schizophrenia, etc. Although understanding the channels through which season of birth affects these outcomes still represents a scientific challenge, in the US winter months are associated with lower birth weight, education, and earnings, while spring and summer are found to be “good” seasons (e.g., Buckles and Hungerman, 2013; Currie and Schwandt, 2013).

Using birth certificate and Census data we provide new evidence on season of birth patterns and correlates with demographic and socioeconomic characteristics among married women, which are absent among unmarried women with no paternity acknowledgement ontheir child’s birth certificate, or among those using assisted reproductive technology (ART)procedures. We argue that these can be explained by season of birth being a choice variablesubject to economic and biological constraints, when women do plan fertility timing. Theplausibility of a demand for season of birth is also documented by the positive averagemarginal willingness to pay for season of birth and spring in particular, which we estimateusing discrete choice experiments in the Amazon Mechanical Turk platform.Plots of first birth prevalence and influenza activity by quarter presented in Figure 1aare consistent with married women choosing a spring birth because their child will be born the farthest from the influenza peak within a year, or summer because there are fewer germsat birth and in the last stage of pregnancy. On the contrary, among unmarried motherswith no paternity acknowledgement, fall (Quarter 4) births are more prevalent, while spring(Quarter 2) births are less prevalent, in spite of facing the same influenza activity as marriedmothers. Moreover, Figure 1b shows that working in particular occupations is correlated with a higher spring birth and lower fall birth prevalence. Thus, influenza and the winterdisease environment are not sufficient to fully explain the observed birth seasonality.

Using US Vital Statistics data on all first singleton births to married women aged 20-45,we show that the prevalence of spring births is related to mother’s age in a humped-shapedfashion, positively related to education and being white, and negatively related to beingHispanic, smoking and receiving food benefits during pregnancy, conditional on gestationweek, state and year fixed effects. However, maternal socioeconomic characteristics do notcorrelate with the probability of having a baby in summer, despite summer being the mostcommon birth season in the US. When focusing on the placebo group of unmarried motherswith no paternity acknowledgement on their child’s birth certificate, our seasonal patternsare muted, consistent with the idea that the children of unmarried women with no stablepartner or long-term relationship are less likely to be planned, and thus it is less likelythat their season of birth is chosen (Almond and Rossin-Slater, 2013; Rossin-Slater, 2017). Indeed, in the US, unmarried women are reported to be more than twice as likely to haveunwanted pregnancies than married women (Finer and Zolna, 2016; Mosher et al., 2012).

We then examine the interaction of a first singleton child’s season of birth with his orher mother’s occupation using data from the American Community Survey. Our findingsreveal that in professions allowing more flexibility in taking time off work and those thathave summer breaks (e.g., among teachers), married mothers are additionally more likely tochoose spring births butnotsummer births, and this holds conditional on age, education,race, ethnicity and state and year fixed effects. This is consistent with the evidence inFigure 1b.

Inspired by Buckles and Hungerman (2013), who recognize that a thorough investigationof preferences for birth timing is an open and fertile challenge for future work, we devisedand ran a series of discrete choice experiments in the Amazon Mechanical Turk marketplace in September 2016 and May 2018, to elicit the willingness to pay for season of birth in two different quarters of the year.1We estimate the average marginal willingness to payfor a spring birth to be 620 USD. We also find that the average marginal willingness to pay (WTP) is larger (about 877 USD) among married mothers aged 20-45, our main groupof analysis in the birth certificate and Census data, whereas among respondents who donot intend to have children the average marginal WTP is much smaller (about 455 USD)and not statistically different from zero, which provides an interesting placebo. Exploringheterogeneity by number of children, we find that our estimate is driven by married mothersaged 20-45 with two or more children (1,100 USD).Using a mixed logit to allow for preference heterogeneity among married mothers aged20-45 in the M-Turk data, we estimate the marginal WTP for spring births for each marriedmother aged 20-45. We then predict the estimated marginal WTP for spring births using maternal characteristics in the same M-Turk data. Assuming transportability from M-Turk to birth certificate data, we use the estimated coefficients on the maternal characteristics to predict the marginal WTP for each married mother aged 20-45 in the latter data. We then investigate the relationship between predicted marginal WTP and spring births in the birthcertificate data. We find that a 1,000 USD increase in the predicted marginal WTP for aspring birth is associated with an increase in the actual probability of giving birth in springof about 15 pp. This finding suggests that average elicited M-Turk responses do correlatewith actual behavior.Our estimated seasonality gaps, between−0.5 pp (Hispanic vs. non-Hispanic) and 0.9pp (received food benefits during pregnancy) in the birth certificate data, and 5 pp by occupation in the Census data, are sizable. Buckles and Hungerman (2013) report a 1 pp difference in teenage mothers and a 2 pp difference in unmarried or non-white mothers between January births and May births, and they interpret these gaps as “strikingly large”compared to the estimated effects of welfare benefits on non-marital childbearing (Rosen-zweig, 1999) or unemployment on fertility (Dehejia and Lleras-Muney, 2004).

1 We thank an anonymous referee for the suggestion to run an additional survey in a different season.

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