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Like many other mammals, humans are seasonal breeders. Seasonality maximizes births at favorable times. Sheep, for example, give birth in spring when it begins to warm up and grass grows. Their breeding season begins five months earlier in the late fall.
The Fall Rut
The seasonal pattern for sheep is similar to that of many other large animals, including humans (1). Fertilization is accomplished in advance of winter. During the winter, many species save energy by moving about as little as possible, whether that means hibernating like bears or resting below ground in burrows where the temperature remains mild and stored food is available.
Human physiology indicates that we are also seasonal breeders. To be more specific, sex hormones, including testosterone, peak in the fall and are at their lowest in the summer. Early researchers made the mistake of focusing mainly on men who have a comparatively weak annual cycle. Women emerge as having a more pronounced increase in testosterone production in the fall that is double their lowest level in the summer (2). This suggests that both sexes would have a higher sex drive in the fall. This is particularly true of women, for whom testosterone is used to boost libido, whereas male testosterone has a less reliable effect on sex drive.
Birth Seasonality
If fall is the season for mating, spring is the season for giving birth. This is also true of humans and there is a distinct (if small) peak in births during the spring (1). This pattern is attributed by researchers to biological factors rather than social patterns like annual vacations.
So far, the cycle is imperfectly understood. One obvious puzzle is the peak in testosterone in the fall that would predict higher conception rates then and higher births in summer. Biologists nevertheless attribute seasonal births to changes in day length and temperature such that some conditions are ideal for conception.
Light, Effects of Day Length, Equinox, Latitude
Evidence from human reproductive physiology showed that when the length of day and night are equal, as at the fall equinox, conditions are optimal for fertilization to occur. It is not known whether this is attributable to increased ovulation by women, or a rise in sperm production by men, or some combination of factors (1).
Similarly, temperatures lingering mostly between 50 and 70 degrees Fahrenheit are optimal for fertilization, and that corresponds to the cooler fall weather in seasonal countries. One way of interpreting the seasonal effects on human fertility is in terms of the suppression of fertilization when it is very hot (in midsummer) or when it is very cold (in winter).
Our species evolved and spent most of its history close to the tropics (which, of course, is one reason that we lost our covering of thick body hair). Under tropical conditions, humans are considerably more fertile than they are at higher latitudes where the climate is more seasonal. I found that countries above 33 degrees latitude have total fertilities less than half that of nations at lower latitudes (1.8 vs. 4.3, 3).
Of course, reproductive physiology is just one factor, another being economic development with high-latitude countries being more economically developed than those lying close to the equator.
This fertility difference is probably unrelated to sex drive. Indeed women in high-latitude countries likely have a higher sex drive because female libido increases with economic development (4).
Paris in Springtime or New England in Fall?
We know that equal length of day and night has a critical effect on reproductive activities of humans and other mammals. Yet, it is the fall equinox rather than the spring equinox that is associated with increasing sex hormones and high fertility. It is not entirely clear why fall is preferred over spring in this respect. Yet, there are at least two plausible biological explanations. One is that with a long stretch of plentiful food animals put on fat stores and are simply in better condition for breeding. The other relates to temperature: At the spring equinox, it often remains quite chilly and sexual ardor is accordingly chilled.
So forget about Paris in the springtime. If you want the perfect context for a (fertile) romantic interlude, go for New England in the fall.
References
1 Roenneberg, T., and Aschof, J. (1990). Annual rhythm of human reproduction: 1. Biology, sociology, or both. Journal of Biological Rhythms, 5, 195-216.
2 Stanton, S. J., Mulette-Gillman, O'D. A., and Huettel, S. A. (2011). Seasonal variation in salivary testosterone in men, normally cycling women, and women using hormonal contraceptives. Physiology and Behavior, 104, 804=808.
3 Barber, N. (2002). On the relationship between fertility and geographic latitude: A cross-national study. Cross-Cultural Research, 36, 3-15.
4 Barber, N. (2008b). Cross-national variation in the motivation for uncommitted sex: The impact of disease and social risks. Evolutionary Psychology, 6, 217-228.
