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.

Keywords: homicide; lunar; time series.

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% confidence band of RR. The moon phases are: new moon (NM), waxing crescent (WaxC), first 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.

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% confidence 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), first quarter (FQ), waxing gibbous (WaxG), full moon (FM), waning gibbous (WanG), last quarter (LQ) and waning crescent (WanC).