Does the Lunar Cycle Affect Birth and Deaths?

Abstract

There is a commonplace notion that full moons affect natality and mortality. To test this theory, we obtain daily births and deaths data from Australia, covering all 10,592 days from 1 January 1975 to 31 December 2003. We find that full moons are not associated with any significant change in the number of conceptions, births, or deaths. Moreover, our standard errors are sufficiently tight to make it possible to rule out even modest positive or negative effects of the lunar cycle.

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The Australian National University
Centre for Economic Policy Research
DISCUSSION PAPER
Does the Lunar Cycle Affect Birth and Deaths?
Joshua S. Gans and Andrew Leigh
DISCUSSION PAPER NO. 532
August 2006
ISSN: 1442-8636
ISBN: 1 921262 02 8
Joshua S. Gans, Melbourne Business School, University of Melbourne
Email: J.Gans@unimelb.edu.au, www.mbs.edu/jgans/
Andrew Leigh, Social Policy Evaluation, Analysis and Research Centre, Research School of Social
Sciences, Australian National University
Email:
andrew.leigh@anu.edu.au, http://econrsss.anu.edu.au/~aleigh/
Acknowledgements
We would like to thank James Downie for inspiring the idea.

ABSTRACT
There is a commonplace notion that full moons affect natality and mortality. To test this
theory, we obtain daily births and deaths data from Australia, covering all 10,592 days
from 1 January 1975 to 31 December 2003. We find that full moons are not associated
with any significant change in the number of conceptions, births, or deaths. Moreover,
our standard errors are sufficiently tight to make it possible to rule out even modest
positive or negative effects of the lunar cycle.
JEL Codes: I12, J13.
Keywords: conceptions, births, deaths, timing, lunar cycle, full moon
ii

1. Introduction
It is frequently suggested that full moons may be associated with higher rates of
conception, births or deaths. While past studies have mostly rejected the presence of full
moon effects (eg. Kelly et al 1996; Owen et al 1998; Chapman and Morrell 2000; Arliss
et al 2005) some have found significant relationships (eg. Lieber 1978; Bhattacharjee
2000). If the lunar cycle has significant effects on conception, birth rates, or death rates,
then doctors would need to take it into account when advising couples hoping to fall
pregnant, while hospital administrators ought to adjust their staffing in maternity wards
and emergency departments. The effect of full moons is therefore more than a curio; if an
effect exists, it has substantial implications for health policy.
2. Does the Full Moon Affect Natality and Mortality?
To test the impact of the full moon on recorded births, we use daily data on the number of
Australian births and deaths. These data are collected by state and territory registries, and
compiled by the Australian Bureau of Statistics. The data cover all 10,592 days from 1
January 1975 to 31 December 2003, encompassing a total of 7,108,772 births and
3,707,146 deaths. We opt to focus on the raw number of births and deaths, rather than on
the rate. This has the advantage that we do not introduce noise into our series through
mis-measurement of the total population, which is only available on a monthly basis.

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Dates are coded as full moons if that day is a full moon, or a full moon occurs on the
preceding or subsequent day (in a typical lunar cycle, 98-100 percent of the moon’s
surface is illuminated for a three-day period).
1
Since full moons occur approximately
once per month, we therefore code about 10 percent of days as “full moons”. Our results
are unchanged if we code only one day per month as a full moon. Table 1 presents
summary statistics.
Table 1: Summary Statistics
Variable Obs Mean Std. Dev. Min Max
Births per
day
10592 671.144 113.872 367 948
Deaths per
day
10592 327.082 43.139 210 490
Full moon 10592 0.101 0.301 0 1
When analysing the effect of full moons on conception, we assume that conception
occurs 266 days prior to birth. There are two limitations in this strategy: many
pregnancies are longer or shorter than 266 days, and not all pregnancies are carried to
term. Nonetheless, using a large dataset covering 29 years of births, the presence of any
substantial “full moon effect” on conception should show up as an increase in the birth
rate nine months later.
We estimate the effect of full moons on conception using ordinary least squares
regression, with robust standard errors. We first present results both without any controls,
and then with indicator variables for the day of the year (366 values), the day of the week
(7 values), and the year (29 values).
1
See discussion at <http://aa.usno.navy.mil/data/docs/MoonFraction.html>

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Our results are shown in Table 2. We find no statistically significant effect of the full
moon on the number of conceptions, births or deaths. Including indicator variables to
take account of day-of-year, day-of-week, and year fixed effects has little effect on the
point estimates, but does reduce the standard errors.
Table 2: Effect of the full moon on conceptions, births and deaths
Ordinary least squares estimates, using daily births and deaths data
Conceptions Births Deaths
Panel A: No controls
Full Moon 0.879 0.906 -0.027
[3.715] [3.710] [1.372]
Observations 10592 10592 10592
R-squared 0.00 0.00 0.00
Panel B: Controlling for day-of-week, day-of-year, and year effects
Full Moon 0.790 0.740 -0.193
[1.349] [1.361] [0.706]
Observations 10592 10592 10592
R-squared 0.89 0.89 0.75
Notes: Robust standard errors in brackets. * significant at 10%; ** significant at 5%; *** significant at 1%.
Full moon is coded as occurring for three days per lunar cycle. Controls in Panel B are day-of-week, day-
of-year, and year fixed effects.
Our 95 percent confidence intervals all span zero, and are quite tight, allowing us to
reject substantial negative or positive effects of full moons. When we control for date
effects, the 95 percent confidence intervals are, for conceptions {-1.854 to 3.433}; for
births {-1.926 to 3.406}; and for deaths {-1.577 to 1.191}.
We can therefore rule out, at the 95 percent confidence level, effects on conceptions,
births or deaths larger than 1/27th of a standard deviation.

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3. Conclusion
A belief that lunar cycles affects fertility, births and deaths remains widespread. Analysis
of births and deaths data for Australia over a 29-year period finds no effect of the full
moon on conception, birth, and deaths. The large sample size allows us to reject even
modest effects of the full moon on these outcomes.

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References
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