Ahhhh, spring. A time for some of us to let our thoughts roam more freely towards love, romance, and sex.
Many of us have heard the saying that, "In the spring a young man's fancy lightly turns to thoughts of love" from the famous poet, Alfred Lord Tennyson. And, just like trees coming alive or flowers waking from their winter dormancy, many of us probably notice that the people around us seem more interested in romance, flirt more, and are more prone to want to engage in sexual activity.
What is the influence of season on love and sex? Well, I'm not so sure about love, and the literature seems pretty scant on that topic. However, here are some interesting findings about fertility. Note that I've generalized a bit, so if you want further details, please do refer to the actual articles.
1) When are rates of sexual behaviour highest? I had a surprisingly hard time finding a good study on this topic. However, one paper (Fortenberry et al, 1997) of adolescent women with sexually transmitted diseases found highest intercourse levels in the spring and summer, and lowest levels in the winter. Interestingly, during the week, most intercourse occurred on Friday and Saturday night, and happened the most infrequently on Sunday!
2) Historically (1778-1940), in northern climates, there is a peak birth rate in late winter (March), and an annual low rate in the summer (June, July and August). This pattern indicates a peak conception period in late spring and early summer, and a low in late summer and fall (Ehrenkranz, 1983).
3) More recently (1974-1982), it seems that the seasonal pattern is a peak birth rate in late summer (August and September), indicating a corresponding peak in conception rates in late fall (November and December; Warren, Gwinn & Rubin, 1986).
4) Menopause onset is significantly higher in winter than spring or autumn, with a smaller peak in summer (Cagnacci, et al, 2005).
5) There seems to be a seasonal trend to menarche (a female's first menstruation), with peaks in summer and winter (Matchock et al, 2004; Gueresi, 1997; Rah et al, 2009). As Rah et al (2009) review, "Seasonal factors, such as length of daylight, ambient temperatures, and the psychological effects of reduced stress and increased relaxation during school vacation, are some suggested explanations for this seasonal variation " (p. 803).
6) Semen quality and quantity also shows seasonal differences. In a review of the existing literature, Levine (1991) found that quantity and concentration of sperm per ejaculation was significantly lower in the summer than in any other season, in places where the summers are hot (e.g., sub-equatorial). Furthermore, Levine reported lowest values of sperm concentration to be clustered toward the late summer and early fall. He proposed that the cause for these decreases was the heat of the summer months, which caused a residual affect through early fall.
7) Related to #5, in more northern countries, where there are large seasonal differences in sunlight, hormonal function and conception rates are lowest during the dark winter months, leading to highest rates of conceptions during the summer, and most births in the spring. Ovulation rates (i.e., women's fertility based on the ovum or egg) may be responsible (Rojansky, et al. 1992).
8) As for men's testosterone levels, there exists considerable controversy. One problem is how it is actually measured (e.g., free testosterone which reflects the levels currently circulating in one's bloodstream, versus total or bound testosterone). One well-done study (Svartberg et al., 2003) found the highest rates during October through December, and "Lowest testosterone levels occurred in months with the highest temperatures and longest hours of daylight" (p. 3099.)
9) Do men view women differently, depending on season? This issue hasn't been well explored (in my opinion), but one study suggests that there is a seasonality effect. Pawlowski and Sorokowski (2008) asked a sample of 114 heterosexual Polish men to judge the attractiveness of photographs of women's faces, breasts, and bodies, at three month intervals from winter 2004 to winter 2005. Photographs of women's breasts and bodies were rated as more attractive during the winter months, but there was no effect for faces. They suggest that, as women cover more of their bodies during the cooler winter months, men's lack of exposure to women's bodies increased attractiveness ratings. Similarly, the abundance of exposed female bodies during the summer causes decreased ratings of the photographs. Presumably, men would see faces just as often regardless of season, which might account for the lack of variation. Other explanations, though, could be seasonal mood changes of participants which could in turn influence perception, and seasonal fluctuations in men's testosterone levels where higher levels of testosterone in winter could induce more positive assessments.
These are only some of the ways that seasonality impacts on sexuality and fertility. Isn't it fascinating?
I pause here to mention one absolutely incredible book on these topics: "The Endocrinology of Social Relationships" edited by Peter Ellison and Peter Gray. I also thank Shannon Flynn, who helped renew my interest in seasonality during her undergraduate thesis research.
Cagnacci, A., Pansini, F.S., Bacchi-Modena, A., Giulini, N., & Mollica, G. (2005). Seasonal onset of the menopause. Maturitas, 51, 393-396.
Ehrenkranz, J. R. L. (1983). Seasonal breeding in humans: Birth records of the Labrador Eskimo. Fertility and Sterility, 40, 485-489.
Fortenberry, J. D., et al, (1997). Weekly and seasonal variation in sexual behaviors among adolescent women with sexually transmitted diseases. Journal of Adolescent Health, 20(6), 420-425.
Gueresi P. (1997). Monthly distribution of menarche in three provinces of north Italy. Annuals Human Biology, 24, 157-168.
Levine, R. J. (1991). Seasonal variation in human semen quality. In A. W. Zorgniotti (Ed.), Temperature and environmental effects on the testis, (pp. 89-96). New York, NY: Plenum Press.
Matchock R. L., et al. (1997). Seasonal rhythms of menarche in the United States: correlates to menarcheal age, birth age, and birth month. Women's Health Issues, 14, 184-92.
Pawlowski, B., & Sorokowski, P. (2008). Men's attraction to women's bodies changes seasonally. Perception, 37, 1079-1085.
Rah, J. H., et al, (2009). Age of onset, nutritional determinants, and seasonal variations in menarche in rural Bangladesh. Journal of Health and Popular Nutrition, 27(6), 802-807
Rojansky, N., et al., (1992). Seasonality in human reproduction: An update. Human Reproduction, 7(6), 735-745.
Svartberg, J., et al. (2003). Seasonal variation of testosterone and waist to hip ratio in men: The Tromsø Study. The Journal of Clinical Endocrinology & Metabolism, 88(7), 3099-3104.
Warren, C. W., Gwinn, M. L., & Rubin, G. L. (1986). Seasonal variation in conception and various pregnancy outcomes. Social Biology, 33, 116-126.