Chronoepidemiological search for circannual changes in the sexual activity of human males

Indirect evidence of circannual (about 1-year) rhythms in the sexual activity of human males was examined utilizing the data of reported rapes in Paris, France, for 1972-78, and Houston, Texas, for 1974-75. In addition, data were available for nearly a 10-year span from Houston, Texas, as the monthly detections of the sexually transmitted diseases (STD) of syphilis and gonorrhea. The results of conventional and cosinor techniques revealed circannual rhythms in the occurrence of rape in both Paris and Houston. With regard to the former sample of 836 reported rapes, cosinor analysis revealed a circannual 0 of July 1, with the 95% CI extending from June 10 to July 20. For Houston, based upon a sample of 1,133 reported rapes, the circannual 0 occurred slightly later, around September 1, with the 95% CI extending from August 15 to September 15. Even though the overall yearly totals of reported rape differed by 4-fold between the 2 cities, the A of the circannual rhythms when expressed relative to the respective M were nearly identical, being 21.6% for Paris and 21.4% for Houston. The data on STD (3,024 cases of primary syphilis and 179,064 cases of gonorrhea), when analyzed by cosinor analysis, also revealed circannual rhythmicity. For gonorrhea, a circannual 0 of October 5, with the 95% CI from July 6 to September 4, was found; for primary syphilis the circannual 0 was somewhat delayed, occurring on November 11, with the 95% CI between October 10 and December 3. These findings, considered together with the reported observations by others of circannual rhythms in plasma testosterone and sexual activity in males as well as the sales of over-the-counter contraceptives, etc., imply that man may be regarded as a seasonal breeder, becoming more sexually active when the photoperiod becomes reduced, i.e. in the autumn. These observations provide new insight into the (chrono)epidemiology of rape and STD.