Abstract
We explored the relationship between month of birth and later outcomes; self-assessed health and years of schooling using ordered probit models with Icelandic data. Results did not confirm an association between month of birth and later outcomes at traditional levels of significance. However, examination of point estimates revealed a relationship that is somewhat consistent with results from the United States, where those born in the first quarter of the year have less educational attainment than those born in other quarters of the year. Of the four possible mechanisms considered, parent’s socioeconomic status is most likely to affect the relationship under study. Future research should explore family background further using data with greater statistical power.
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Notes
Data consisted of all men born 1930–1949 in the 1980 census 5 percent sample. Sample size was 312,718 for 1930–1939 cohort and 457,181 for 1940–1949 cohort [Angrist and Krueger 1991]. Buckles and Hungerman used data consisting of males born between 1944–1955 in the 1980 census. Sample size was 1,090,826 (2013).
The beginning and end of compulsory schooling are both a function of date of birth, but in different ways, leading to variability in the length of compulsory schooling by date of birth. Most states in the United States require students to enter school in the calendar year in which they turn 6. In states with a December 31 birthday cut-off, those born late in the year are relatively young for their grade. That way, children born in the fourth quarter enter school at age 5 ¾, while those born in the first quarter enter school at age 6 ¾. As compulsory schooling laws typically require students to remain in school until their 16th birthdays, these groups of students will have had different amounts of schooling when they reach the legal dropout age. This is the rationale for Angrist and Krueger‘s approach in 1991.
It may be of interest to know that the reason for the quarter of birth division in Angrist and Krueger‘s study is that data for seperate months was not available from the US Census. Their analysis was thus constrained by using quarter of birth instead of month of birth that could be less sensitivite to proposed variability in education.
Buckles and Hungerman also used quarter of birth from census data for years 1960 and 1980 when studying mother characteristics, years of schooling, and wages (male sample). Sample sizes varied from 927,954–1,090,826 depending on dependent variable being observed. Their data from Natality Files, 1989–2001 included 52,041,054 observations that they used to study variations in mother characteristics by month of birth.
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Appendices
Appendix A
Seasonal patterns in birth
Pronounced and persistent seasonal patterns in births have been observed across human populations [Lam and Miron 1991, 1994]. These seasonal patterns have raised questions about the behavioral and biological determinants of reproduction. The seasonality of births varies between populations. In Northern Europe most births occur in the spring (March−April) whereas birth rates are lowest in the autumn (October−December). In the United States, by contrast, most births occur in the summer and early autumn (September peak) and the minimum is in spring (April−May) [Lam and Miron 1994].
Figure A1 shows birth frequency in Iceland for years corresponding to the data used in this analysis separated into two periods to show more precisely the pattern in birth frequency. This pattern is in line with patterns as described by Lam and Miron for the Northern Hemisphere with most births occurring in April−May and lowest birth rates close to the end of the year.
Birth frequency per month in Iceland [Statistics Iceland 2010b].
The causes of these seasonal variations are not fully understood. Possible explanations may partly be linked to seasonal fluctuations in environmental factors, such as temperature and photoperiod (daily exposure to light). The effect of temperature on conception may result from changes in coital frequency or may reflect direct physiological effects. Lam and Miron [1994] found mixed evidence on the role of temperature in explaining birth seasonality and suggest that other factors play an important role in birth seasonality. The Southern US pattern appears to be heavily influenced by summer heat, but the Northern European pattern appears to have little to do with temperature.
One proposed explanation for the summer heat effect is related to a socioeconomic differential in birth seasonality [Kestenbaum 1987]. Kestenbaum found more pronounced birth seasonality in lower socioeconomic groups (measured by parental income) and interprets that as a reflection of the greater ability of those better-off to control climate, for example, through air-conditioning. In a study by Buckles and Hungerman [2013], summer weather is claimed to differentially affect fertility patterns across socioeconomic groups with those in lower groups being more affected than others. This is in contrast to other results that have found more pronounced birth seasonality within higher socioeconomic groups as measured by education, age, and marital status [Bobak and Gjonca 2001], which may indicate that those in higher socioeconomic groups have more control of their reproduction by individual choices. These theories are contradictory as to whether birth seasonality is more pronounced in higher or lower socioeconomic groups. At the same time they emphasize different explanations to birth seasonality; temperature, and preferences.
A photoperiod is often linked to weather conditions in this literature but there is little evidence on the proposed effect of a photoperiod on birth seasonality. Other proposed explanations to birth seasonality are seasonality in pregnancy loss [Weinberg et al. 1994] or cultural factors like the choice of the time of pregnancy [Basso et al. 1995]. Seasonal patterns in marriages, holidays, temporary migration, and economic variables, including agricultural cycles are among speculations that have been documented [Lam and Miron 1991]. Religious festivals, business cycles, and the existence of occupations requiring the temporary absence of the husband have also been cited as determinants of birth seasonality [Condon and Scaglion 1982].
Appendix B
Regression of quarters of birth on years of schooling
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Ólafsdóttir, T., Ásgeirsdóttir, T. Does Month of Birth Affect Individual Health and Educational Attainment in Iceland?. Eastern Econ J 41, 329–345 (2015). https://doi.org/10.1057/eej.2014.20
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DOI: https://doi.org/10.1057/eej.2014.20