No. 78: Nov-Dec 1991
Using 2,131 acts of histility recorded over the last 3500 years, G. Schreiber et al have shown that these conflicts did not begin at random. Instead onsets of hostility are nicely correlated with the number of hours of sunlight in the day each war began.
"In the Northern Hemisphere, latitudes 30-60° N., the annual rhythm in the opening dates of wars shows a peak in August and a nadir in January (a in the figure). An inverse pattern in the annual rhythm of wars with a peak in December-February and a nadir in July was found in the Southern Hemisphere latitudes 30-60° S (c in the figure)....The results in the Northern Hemisphere suggest that there is a phase-shift of about one month between the two rhythms. We found a constant rate of acts of hostility throughout the year around the line of the Equator (b in the figure)."
(Schreiber, Gabriel, et al: "Rhythms of War," Nature, 352:574, 1991.)
Comment. From the curves, it appears that inhabitants of the Northern Hemisphere are about 20 times more bellicose than those below the Equator (a population effect?).
Reference. The cyclicity of human behavior requires several categories in the catalog volume Biological Anomalies: Humans I, notably BHB8. Ordering information here.