Seasonal variation in birth rates is a ubiquitous property of human populations. Although the general birth rate is dependent on relatively few parameters—sexual behavior, zygote survival, and the number of ova released—the quantitative influence of the effects of each is difficult to estimate. Research has been directed more towards seasonal variation in physiology and less towards behavioral effects such as preferences for birthing in particular seasons. This may be because alongside seasonal variation in birth rate there is also seasonal variation in the multiple birth rate, which necessarily depends on the same physiological factors as the birth rate, suggestive of a link. Here we use a simulation that follows females through their reproductive lives, replicating the incidence of singleton and twin births that arise from single and double ovulations. We use the simulation to quantify the impact of seasonal variation in the physiological variables that affect birth rate. We can generate the seasonal variation in twinning rates observed in European populations, but this does not generate the magnitude of the observed seasonal variation in birth rate. Our simulation shows that, despite correlations between twinning rate and general birth rate, seasonal changes in sexual behavior are necessary to explain the magnitude of the seasonal variation in the general birth rate in humans.

出生率的季节性变化是人类普遍存在的特性。尽管一般出生率取决于相对较少的参数——性行为、受精卵存活率和释放的卵子数量——但每个参数的影响的定量影响很难估计。研究更多地针对生理学的季节性变化，而不是行为影响，例如对特定季节分娩的偏好。这可能是因为除了出生率的季节性变化外，多胞胎出生率也存在季节性变化，这必然取决于与出生率相同的生理因素，这表明存在联系。在这里，我们使用了一个模拟，跟踪女性的生殖生活，复制了由单排卵和双排卵引起的单胎和双胞胎分娩的发生率。我们使用模拟来量化影响出生率的生理变量的季节性变化的影响。我们可以生成在欧洲人群中观察到的双胞胎率的季节性变化，但这并不能产生观察到的出生率季节性变化的幅度。我们的模拟表明，尽管双胞胎率和一般出生率之间存在相关性，但性行为的季节性变化对于解释人类一般出生率的季节性变化幅度是必要的。