Lunar cycle in homicides: A population-based time series study in Finland

Abstract

Objective
To test whether homicides in Finland vary according to moon phases.

Design
A time series study.

Setting
Finland.

Participants
6808 homicides committed in 1961–2014.

Outcome
Daily numbers of homicides.

Method
The daily numbers of homicides were related to eight moon phases and the illuminated percentage of the moon disc using negative binomial regression. To identify lunar patterns, piecewise linear terms of lunar days were used, together with changes from one moon phase to another. Homicides were similarly regressed on quintiles of the illuminated percentage of the moon disc. A periodic term of length 29.53 days was included to detect cyclic patterns. The results were adjusted for sex, age, secular trend, distance from the moon, seasons, weekday, major holidays and temperature.

Results
During the full moon, 15% less homicides were committed than during the new moon (RR 0.85; 95% CI 0.77 to 0.94) and 86% less during the lightest quintile of illumination compared with the darkest quintile (RR 0.14; 95% CI 0.04 to 0.50). Adjustments did not change the results. Piecewise linear regressions showed a significant decline in homicides at the full moon and a rise thereafter. The full moon drop in homicides was directionally similar for seasons, weekdays, sex, age and time periods, and it was particularly pronounced in the early part of period studied (1961–1974). Periodic regression showed a regular cyclic pattern of length 29.53 days (p~0.035).

Conclusions
Contrary to current scientific opinion, an association exists between moon phases and homicides, and contrary to what has been previously assumed, homicides declined during the full moon, especially in earlier decades. However, the causality of the association remains elusive.

Full text

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NäyhäS. BMJ Open 2019;9:e022759. doi:10.1136/bmjopen-2018-022759
Open access
Lunar cycle in homicides: a population-
based time series study in Finland
Simo Näyhä
To cite: NäyhäS. Lunar
cycle in homicides: a
population-based time series
study in Finland. BMJ Open
2019;9:e022759. doi:10.1136/
bmjopen-2018-022759
►Prepublication history and
additional material for this
paper are available online. To
view these les, please visit
the journal online (http:// dx. doi.
org/ 10. 1136/ bmjopen- 2018-
022759).
Received 9 March 2018
Revised 23 October 2018
Accepted 5 December 2018
Faculty of Medicine, Center for
Environmental and Respiratory
Health Research, University of
Oulu, Oulu, Finland
Correspondence to
Professor Simo Näyhä;
simo. nayha@ oulu. 
Research
© Author(s) (or their
employer(s)) 2019. Re-use
permitted under CC BY-NC. No
commercial re-use. See rights
and permissions. Published by
BMJ.
ABSTRACT
Objective To test whether homicides in Finland vary
according to moon phases.
Design A time series study.
Setting Finland.
Participants 6808 homicides committed in 1961–2014.
Outcome Daily numbers of homicides.
Method The daily numbers of homicides were related to
eight moon phases and the illuminated percentage of the
moon disc using negative binomial regression. To identify
lunar patterns, piecewise linear terms of lunar days were
used, together with changes from one moon phase to
another. Homicides were similarly regressed on quintiles
of the illuminated percentage of the moon disc. A periodic
term of length 29.53 days was included to detect cyclic
patterns. The results were adjusted for sex, age, secular
trend, distance from the moon, seasons, weekday, major
holidays and temperature.
Results During the full moon, 15% less homicides were
committed than during the new moon (RR 0.85; 95% CI
0.77 to 0.94) and 86% less during the lightest quintile of
illumination compared with the darkest quintile (RR 0.14;
95% CI 0.04 to 0.50). Adjustments did not change the
results. Piecewise linear regressions showed a signicant
decline in homicides at the full moon and a rise thereafter.
The full moon drop in homicides was directionally similar
for seasons, weekdays, sex, age and time periods, and
it was particularly pronounced in the early part of period
studied (1961–1974). Periodic regression showed a
regular cyclic pattern of length 29.53 days (p~0.035).
Conclusions Contrary to current scientic opinion, an
association exists between moon phases and homicides,
and contrary to what has been previously assumed,
homicides declined during the full moon, especially in
earlier decades. However, the causality of the association
remains elusive.
INTRODUCTION
A number of studies have tried to identify
lunar variations, the so-called ‘Transylvania
effect’1 in various types of human activity such
as aggressive behaviour,2 assaults, crime,3 4
homicides,5–9 traffic accidents,10 suicides9 11
and mental disorders.12 One line of thought
has been that gravitational forces, which are
greatest during the full moon and the new
moon, would cause cyclic fluid shifts between
body compartments and thereby trigger
emotional disturbances, suicides and aggres-
sive behaviour in predisposed individuals, as
stipulated in the theory of ‘biological tides’.7
Other theories have considered moon-re-
lated variations in electromagnetic fields,13
weather14 and illumination15 16 as potential
factors, which could affect human behaviour
and cause adverse health effects.
While the belief in lunar effects on humans
is reportedly common, especially among
healthcare personnel12 17 18 and police,19 it
is not well supported by empirical evidence.
Studies claiming such effects are said to be
poorly conducted due to questionable data
analysis, insufficient sample size, too short
periods studied, data dredging or uncon-
trolled confounding by season, weekdays or
holidays,12 20–24 and their findings have not
been replicated in other populations.6 19 22 A
comprehensive review14 concluded that not
even in theory can lunar factors interfere with
human behaviour and discouraged further
studies, as did some reviews and meta-anal-
yses.19 20 Thus, human beings would be imper-
vious to lunar effects,25 perhaps excepting
the trivial fact that some activities are easier
in moonlight14 and accidents more likely to
occur during the dark new moon nights.26
While many earlier works have studied
potential effects of the full moon on aggres-
sion and homicides,5–9 27 few studies have
addressed this particular topic nowadays,3 12 all
with negative or equivocal results. The present
paper sets out to retest the hypothesis that no
association exists between the lunar cycle and
Strengths and limitations of this study
►The analyses were controlled for potential con-
founding by sex, age, secular trend, distance
from the moon, seasons, weekdays, holidays and
temperature.
►The data comprised a long time series from a na-
tional population.
►Limitations include that the exact time of death
was not known, and homicides could not be broken
down topremeditated murders and manslaughters,
which may have different patterns.
►The association observed does not imply a causal
relation between the moon phases and homicides.
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2NäyhäS. BMJ Open 2019;9:e022759. doi:10.1136/bmjopen-2018-022759
Open access
homicides. The data comprise a 54-year time series of
homicides in Finland, which is subjected to linear spline
and periodic regressions to identify any lunar patterns.
METHODS
Data
The daily numbers of homicides in Finland during
the period 1961–2014, classified by sex and age group,
were obtained from Statistics Finland, the study period
consisting of 668 lunar synodic cycles. The cases were
defined as those deaths where homicide was recorded as
the underlying cause (in 1961–1968 ICD-7 codes E964
and E980–E984; in 1969–1986 ICD-8 codes E960–E969;
in 1987–1995 ICD-9 codes E960–E969; in 1996–2014
ICD-10 codes X85–Y09 and Y87.1). Annual populations
were obtained from official statistical sources. Informa-
tion on mean daily temperatures in 13 weather stations
representing all regions of the country (South: Helsinki,
Juupajoki, Kaarina, Lahti; Central: Seinäjoki, Jyväskylä;
East: Mikkeli, Joensuu, Kuopio, Kajaani; North: Oulu,
Rovaniemi, Sodankylä) was obtained from the Finnish
Meteorological Office, missing observations (0.1%)
being estimated by cubic spline regression with 3 df. Due
to small daily numbers, no analysis by region could be
conducted, and instead daily temperatures were averaged
over the weather stations.
Statistical analysis
The daily counts were linked to eight lunar phases (new
moon, waxing crescent, first quarter, waxing gibbous, full
moon, waning gibbous, last quarter, waning crescent).
The approximate days of the lunar synodic cycle were
calculated based on radians (1 lunar day=2π/29.53).
Since the lunar synodic cycle is slightly irregular, with vari-
ation from 29.18 to 29.93 days, the illuminated percentage
of the moon disc (0%‒100%) was used as an additional
explanatory variable. All measures were obtained using
the lunar package available in the R software.
Crude homicide mortality by lunar phases was first
expressed as the number of homicides per 100 000
person-years. Then the daily counts were regressed on
eight lunar phases (a categorical variable), adjusting
for sex, age (in classes 0˗14, 15˗64, 65+ years), secular
trend (a third-degree polynomial), distance of the
moon from the earth (in classes apogee, far, average,
near, perigee), seasons (Winter: December to February;
Spring: March to May; Summer: June to August; Autumn:
September to November), weekdays, those public holi-
days which showed association with homicides (New
Year, Midsummer Eve, Midsummer Day, Christmas Eve)
and temperature. To allow for extra-Poisson variation
caused by more than one person dying in the same inci-
dent, negative binomial regression was used. The good-
ness-of-fit test (the goodfit function in R) first showed a
poor fit of daily homicides to negative binomial distribu-
tion (p~0.000), but after removing two outliers (8 and
10 school shootings committed by a single perpetrator
each), the fit improved (p~0.124). All regressions were
run with and without exclusion of these outliers, but the
results were practically identical, and only results without
exclusions are shown. The link function used was loga-
rithmic, and the results were expressed as rate ratios (RR)
and their 95% CIs.
The lunar association of homicides was also examined
by regressing daily homicides on linear splines of lunar
synodic days, using midpoints of moon phases as knots.
This analysis retained the continuity of the explanatory
variable (lunar day) but allowed for non-linearity of the
association and also enabled piecewise comparisons of
regression slopes and their changes from one phase to
another. Homicides were similarly regressed on linear
splines of the illuminated proportion of the moon disc,
using quintile points of the illuminated percentage as
knots, with a further study of changes of piecewise regres-
sion slopes between the quintiles. To reveal any regular
lunar cyclicity in homicides, the daily counts were treated
as a time series of 19 723 days, fitting a periodic term
of 29.53 days in length, together with other periodic-
ities (1 year, ½ year, 1/3 year, 1/4 year, 1 week and 1/2
week). The calculations were performed using R, V.3.1.0
(R Development Core Team, 2014, available at: http://
www. R- project. org) and Stata, V.11 (StataCorp, College
Station, Texas, 2009).
Patient and public involvement
No patients were involved in this study.
RESULTS
Descriptive data
The mean daily number of homicides was 0.35 (range
0–10) and mortality was 2.54 deaths per 100 000 person-
years (table 1). Altogether, 751 homicides were committed
during the full moon, compared with 831–895 during
other lunar phases, with respective mortalities of 2.24 and
2.48–2.68 per 100 000 person-years. The distribution of
days according to the illuminated portion of the moon
disc was very uneven, with strong accumulations on the
lightest and darkest ends of the illumination scale (figure
SF1 in the online supplementary file). During full moon
days, the moon was almost fully illuminated (96%–100%)
and during new moon days, the moon was almost fully
dark (0%–4%), while during all other phases, the illumi-
nated percentage was distributed over broader ranges,
that is, 26%–38% (table 1).
Homicides and moon phases
The left-hand panel of figure 1 compares crude homi-
cide mortality by moon phases using rate ratios obtained
from negative binomial regression with the moon phase
as a categorical explanatory factor. Compared with the
empty model (no explanatory factors), the p value from
the likelihood ratio test was 0.028, indicating some differ-
ence in mortality between the moon phases. In particular,
mortality during the full moon phase was 15% lower than
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Table 1 Moon phases and homicides
Moon phase
(approximate
days*)
Percentageof
moon disc
illuminated
Temperature
(°C)† Days(n)
Homicides per
phase(n)
Mean number of
homicides per
day
Person-years ×
10 000
Mortality/
100 000 person-
years
Adjusted rate ratio (RR)‡
RR 95% CI
New moon (27.7–
1.9)
0–4 3.1 2467 883 0.358 3349.3 2.64 1.00
Waxing crescent
(1.9–5.5)
4–30 3.1 2465 889 0.361 3346.5 2.66 1.01 0.92to1.11
First quarter
(5.5–9.2)
30–68 3.0 2467 857 0.347 3349.3 2.56 0.97 0.88to1.07
Waxing gibbous
(9.2–12.9)
68–96 3.0 2461 895 0.364 3341.1 2.68 1.01 0.92to1.12
Full moon (12.9–
16.6)
96–100 2.8 2466 751 0.305 3347.9 2.24 0.85 0.77to0.94
Waning gibbous
(16.6–20.3)
70–97 3.0 2464 846 0.343 3345.2 2.53 0.96 0.87to1.06
Last quarter (20.3–
24.0)
32–70 2.9 2465 831 0.337 3346.5 2.48 0.94 0.86to1.04
Waning crescent
(24.0–27.7)
4–32 3.2 2468 856 0.347 3350.6 2.55 0.97 0.88to1.07
P~§ 0.015
Total 3.0 19 723 6808 0.345 26 777.1 2.54
*Days 0.0–29.5 calculated based on radians with the full moon cycle of 2π and 1 day corresponding 0.213 radians.
†Mean daily temperature.
‡From negative binomial regression adjusting for sex, age, secular trend (third-degree polynomial), distance from the moon, seasons, weekdays, public holidays (New Year, Midsummer Eve, Midsummer
Day, Christmas Eve) and temperature.
§From likelihood ratio test.
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that during the new moon (RR 0.85, 95% CI 0.77 to 0.95),
while during no other phase did RR deviate from the
reference level. The adjusted RRs (table 1, table ST1 in
the online supplementary file) remained almost similar
to the unadjusted ones.
Figure SF2 in the online supplementary file reviews
the consistency of the lunar pattern of homicides by
subgroups and shows an almost invariably low RR during
full moon across years, seasons, weekdays, sex and age.
The homicide dip during the full moon was particu-
larly steep in 1961˗1974 (RR 0.77; 95% CI 0.61 to 0.96)
compared with other periods, and it was also low among
women (RR 0.72; 95% CI 0.60 to 0.88), in spring (RR
0.79; 95% CI 0.64 to 0.97), autumn (RR 0.78; 95% CI
0.63 to 0.97) and in days other than weekend days (RR
0.82; 95% CI 0.71 to 0.95). However, all likelihood ratio
tests for interaction between lunar phase and these
stratification factors were non-significant at a 0.05 level,
indicating a failure to reject the null hypothesis that the
overall patterns are uniform over the strata.
Table 2 shows the piecewise regression slopes (change
of RR per lunar day) and their changes from one moon
phase to another. Just before the full moon, that is, on
approximate lunar days 11–15 (waxing gibbous to full
moon), the homicide rate declined by 6% per 1 lunar
day (RR 0.94; 95% CI 0.91 to 0.98), and immediately after
that, that is, on days 15˗19 (full moon to waning gibbous)
it rose again by 5% per day (RR 1.05; 95% CI 1.01 to 1.09).
A different parametrisation in the right-hand column
of table 2 shows that during the lunar days 11˗15, the
curve turned down by 8% (RR 0.92; 95% CI 0.86 to 0.99)
compared with the preceding interval (days 7–11), and
the upturn that followed during the next interval was
11% (RR 1.11; 95% CI 1.04 to 1.20).
Homicides and the illuminated proportion of the moon
Homicide mortality is also shown in relation to the illumi-
nated proportion of the moon disc in the form of linear
splines of the illuminated percentage (right-hand panel
of figure 1). Compared with the darkest stage (0% illu-
minated), the crude RRs remained relatively unchanged
over the quintiles I˗IV of illumination but declined
steeply during the lightest quintile, down to 0.84 (95% CI
0.77 to 0.92) at the fully illuminated moon.
The piecewise regression slopes (change of RR per one
percentage unit illuminated) in table 2 were not signifi-
cant in quintiles I–IV, but the slope parameter was signifi-
cantly small in the lightest quintile (RR 0.14; 95% CI
0.04 to 0.50), indicating a steep decline in homicides,
with the RR 90% smaller than that in the preceding quin-
tile (RR 0.10; 95% CI 0.02 to 0.53).
Periodic regression
Table 3 summarises the periodic regression of homicides
fitting the first-order sinusoid terms for the lunar synodic
cycle together with other periodicities and adjusting for
the secular trend. The periodicities of 6 and 3 months
proved insignificant at 0.05 level and were omitted. The
periodic components are shown graphically in figure
SF3 in the online supplementary file. The pattern is
dominated by wide-amplitude 7 and 3.5-day cycles, corre-
sponding to their large positive cosine and large negative
Figure 1 Rate ratio (RR) for homicides in Finland, 1961–2014, by moon phases (left) and by percentage of the illuminated
portion of the moon disc (right). The shaded area shows the 95% condence band of RR. The moon phases are: new moon
(NM), waxing crescent (WaxC), rst quarter (FQ), waxing gibbous (WaxG), full moon (FM), waning gibbous (WanG), last quarter
(LQ) and waning crescent (WanC). P values from likelihood ratio test: 0.028 for moon phases, 0.022 for percentage of moon disc
illuminated.
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sine terms (table 3), with somewhat lower amplitude
1-year and 4-month cycles. The 29.53-day lunar cycle is
also significant and most influenced by its negative sine
term. The model-predicted composite pattern of homi-
cides incorporating all significant periodicities is illus-
trated in figure 2, together with the lunar periodicity
alone. The pattern is shown for the 538th lunar cycle
(14 August to 11 September 2004), during which homi-
cide mortality was close to the average mortality during
the whole study period. The lunar cycle is overshadowed
by the weekly swings but is still significant, the model-es-
timated peak and trough locating on approximately
second and 17th lunar days. The trough of the estimated
mortality curve was 9% lower than the estimated peak
(95% CI 5% to 13%).
DISCUSSION
Most studies on lunar effects on humans have produced
negative results, including those focused on homi-
cides,5 6 9 19 20 28 although some negative findings may
be due to type II error.29 Positive findings have been
reported for aggravated assaults,2 crimes,30 poison centre
calls,31 extremely violent behaviour17 and general prac-
tice consultations.1 The observations reported as positive
have been attributed to selective sampling, erroneous
definitions of lunar phases, too short periods studied,
flawed data analysis, confounding, type I error or data
dredging.6 14 20 The present study based on a time series
of 19 723 days and 668 lunar cycles found an unequivocal
lunar pattern in homicides that was not confounded by
sex, age, secular trend, distance from the moon, seasons,
weekdays, holidays or temperature, and the finding was
consistent across relevant subgroups. The analysis also
revealed a regular lunar pattern in homicides. Since
the data comprised the entire national population, they
cannot be distorted by biased sampling. The results
do not support the notion emanating from previous
research that no association exists between the moon and
homicides.
The study of Lieber and Sherin,7 based on two inde-
pendent samples from the USA, is sometimes referred to
as the only one to have observed a statistically significant
variation of homicides by lunar phases.20 The authors
linked homicides to lunar phases using time of injury,
but if time of death was used instead, the association
disappeared.8 Thus, the present study is the first one to
show that homicidal deaths vary according to the moon
phases. The pattern was still unexpected, since homicides
reduced during the full moon while a bimodal rise during
the full moon and the new moon was found in the above
study.7 Violent behaviour17 and aggravated assaults2 have
also been reported to increase during the full moon,
Table 2 Adjusted* rate ratios (RR) and their 95% CIs from piecewise negative binomial regression of homicides on lunar days
and on percentage of moon disc illuminated
RR† 95% CI
Relative change in RR versus
preceding interval
RR 95% CI
Intervals between midpoints (day) of moon phases
New moon (0) to waxing crescent (3.7) 1.00 0.95 to 1.05 –
Waxing crescent (3.7) to rst quarter (7.4) 0.98 0.94 to 1.02 0.98 0.91 to 1.06
First quarter (7.4) to waxing gibbous (11.1) 1.02 0.98 to 1.06 1.04 0.97 to 1.12
Waxing gibbous (11.1) to full moon (14.8) 0.94 0.91 to 0.98 0.92 0.86 to 0.99
Full moon (14.8) to waning gibbous (18.5) 1.05 1.01 to 1.09 1.11 1.04 to 1.20
Waning gibbous (18.5) to last quarter (22.1) 0.98 0.94 to 1.02 0.93 0.87 to 1.00
Last quarter (22.1) to waning crescent (25.8) 1.03 0.98 to 1.07 1.05 0.98 to 1.13
Waning crescent (25.8) to new moon (29.5) 1.00 0.95 to 1.05 0.97 0.90 to 1.05
P~‡ 0.047 0.047
Quintiles of illuminated percentage of moon disc
I (0.0% to 9.6%) 1.10 0.31 to 3.88 –
II (9.6% to 34.5%) 0.78 0.48 to 1.27 0.71 0.14 to 3.56
III (34.5% to 65.5%) 0.99 0.67 to 1.45 1.26 0.57 to 2.80
IV (65.5% to 90.5%) 1.34 0.82 to 2.19 1.36 0.61 to 3.00
V (90.5% to 100.0%) 0.14 0.04 to 0.50 0.10 0.02 to 0.53
P~‡ 0.011 0.011
*Adjusted for sex, age, secular trend (third-degree polynomial), distance from the moon, seasons, weekdays, public holidays (New Year,
Midsummer Eve, Midsummer Day, Christmas Eve) and temperature.
†Relative change per 1 day, or relative change per one percentage unit of illumination.
‡From likelihood ratio test.
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which is relevant here, since violence and assaults repre-
sent the same criminal act as homicide, except that
the victim survives. No previous study has found any
significant decline in homicides during the full moon,
although such declines have been reported for emer-
gency contacts,24 trauma,26 traffic accidents10 and alcohol
intake.32
Limitations of this study include the fact that homi-
cides were assigned to moon phases using the day of
death, while factors decreasing homicides during the full
moon would start to influence some time before that.
The resulting inaccuracy cannot be great, since approx-
imately 85% of homicide victims in Finland are found
dead or die within 24 hours.33 Comparable figures have
been reported from the USA6 34 and Europe.35 An addi-
tional source of error is that the exact time of the day
of the homicide was not known, since potential effects
of moonlight should be different by night and day.36 A
further limitation is that homicides could not be broken
down to premeditated murders and manslaughters, the
timing of which may be influenced by different factors.
Even though an association was found between the moon
phases and homicides, the study design and available data
do not allow any causal conclusions to be drawn.
Any positive findings based on empirical data may
be due to type I error, that is, a departure from the
null hypothesis when no true difference exists. This is
sometimes offered as an explanation for allegedly positive
findings in studies claiming lunar effects, especially those
failing to fulfil the criteria for causality, for example,
consistency of the finding across subgroups of the data.
The credibility of the present finding is corroborated by
the similarity of the pattern and its similar directionality,
in successive time periods, seasons, weekdays, in men and
women and different age groups, with the homicide drop
mostly occurring during the full moon. The decline in
homicides during the full moon was confirmed by phase-
by-phase analysis and by analysis of successive illumina-
tion quintiles. The possibility of a coincidental finding
is also reduced by the long time period, relatively large
sample size and the largest number of lunar synodic
cycles ever studied. In theory, it is possible that small
sample sizes in many previous studies have led to type II
errors, preventing any recognition of true effects.29
While investigations reporting an increase in adverse
human behaviour during full moon mostly refer to some
geophysical explanation,14 those observing a decrease in
such phenomena during full moon have had more diffi-
culty to explain their findings. Thus, studies noting a
decrease in emergency contacts24 and traffic accidents10
during the full moon did not contemplate the underlying
reasons at all, while de Castro and Pearcey32 assumed that
the decrease in alcohol intake with increasing moonlight
could be attributed to some moon-related biological
Table 3 Periodic regression of daily homicides, 1961‒2014
Term Coefficient z
P value
foradding
terms*
Secular trend 0.000
x −0.631 −0.320
x2−25.944 −13.455
x3−5.127 −2.671
12 months 0.057
Sin(x) −0.018 −0.964
Cos(x) −0.040 −2.198
4 months 0.021
Sin(x) −0.051 −2.780
Cos(x) −0.005 −0.281
7 days 0.000
Sin(x) −0.051 −2.712
Cos(x) 0.172 9.564
3.5 days 0.000
Sin(x) −0.060 −3.276
Cos(x) 0.100 5.474
29.53 days 0.035
Sin(x) −0.038 −2.085
Cos(x) −0.028 −1.540
x=days 1…19 723.
*From likelihood ratio test.
Figure 2 Regular periodicity of daily homicides in
Finland assessed by periodic regression. Model-predicted
numbers of homicides during the lunar cycle 14August
to 11September 2004 are shown. Thin line describes the
multicycle periodicity composed of the lunar cycle (29.53
days; p~0.035), 12 months (p~0.057), 4 months (p~0.021), 7
days (p~0.000) and 3.5 days (p~0.000). Thick lines represent
the lunar periodicity alone (dashed lines: 95% condence
band). White circles indicate the observed mean numbers
and mortality of homicides during the entire study period.The
moon phases are: new moon (NM), waxing crescent (WaxC),
rst quarter (FQ), waxing gibbous (WaxG), full moon (FM),
waning gibbous (WanG), last quarter (LQ) and waning
crescent (WanC).
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NäyhäS. BMJ Open 2019;9:e022759. doi:10.1136/bmjopen-2018-022759
Open access
rhythm. A Dutch study26 noting a statistically significant
decline of 2% in traffic accidents during the full moon,
attributed this to better visibility due to moonlight, an
explanation regarded as trivial by some.14 As a brighter
environment may deter criminal acts,16 moonlight could
have played a role in the prominent full moon dip in
homicides in 1961˗1974 when one-half of Finns lived in
countryside compared with one-fourth nowadays when
most people live in cities and an artificially lit environ-
ment. However, the causal role of moonlight remains
unclear also because most homicides in this country are
committed indoors,33 and the actual lighting conditions
in each case were not known.
The seasonality of homicides with a summer peak,
potential bimodality and the changes in the pattern since
the 1870s has been reported previously,37 38 but no study
has decomposed the annual variation to shorter cycles.
The concentration of homicides to weekends and the
association with alcohol consumption is well known.39
The present study controlled for the effect of weekdays
but not for alcohol consumption in lack of daily data. Any
regular 3.5-day cycle in homicides has not been reported
previously and it warrants further study.
Why homicides decreased during the full moon is not
easily explained. One might speculate that the full moon,
so clearly perceived and distinguished from other moon
phases by virtue of its fully illuminated disc and short
duration, may have some superstition-based meaning
in peoples’ minds that refrains potential lunar phobic
perpetrators from committing the act. The belief that the
moon affects human behaviour is common among health-
care personnel, police17–19 and the general public.40 The
victim’s behaviour might play a role, too. As in most cases,
the victim and the perpetrator know each other and have
been in dispute before,33 potential homicide victims
who feel themselves threatened may avoid moonlight to
protect themselves, or they may believe that something
unfortunate could happen during the full moon. Some
might see the drop in homicides during the full moon
as an atavistic remnant from the animal kingdom where
certain prey animals suppress their activity in moonlight
to hide themselves from predators, or perhaps, their
enhanced visual acuity in moonlight would help them
detect predators.41 However, factors other than moon-
light may play a role as well.
CONCLUSIONS
Homicides in Finland follow a detectable and regular
cyclic pattern with a decrease during the full moon.
This decrease, applied to all moon phases, would imply
a decrease of approximately 20 homicides in an average
year, that is, a decrease of 0.38 homicides per 100 000
person-years, compared with the average of 2.54/100 000
during the entire study period. Irrespective of what the
underlying reasons may be, the prospects for prevention
are limited, as most people dwell in towns nowadays and
have less contact with the natural environment. Perhaps,
this was reflected in little variation in homicides in the
2000s. In any case, the finding challenges the current
scientific opinion that the lunar cycle and homicides are
unrelated, and it questions the widely held belief that the
full moon may provoke violent behaviour.
Contributors SN conceived the study idea, did all data analyses and wrote the
manuscript.
Funding Institutional funding.
Competing interests None declared.
Patient consent Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement The data are condential and cannot be shared.
Open access This is an open access article distributed in accordance with the
Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which
permits others to distribute, remix, adapt, build upon this work non-commercially,
and license their derivative works on different terms, provided the original work is
properly cited, appropriate credit is given, any changes made indicated, and the use
is non-commercial. See: http:// creativecommons. org/ licenses/ by- nc/ 4. 0/.
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