| Mammalian reproduction is strongly affected by photoperiod and ambient temperature. Although human fertility is seasonal, there has been little systematic investigation of cross-national variation in fertility as a function of latitude and climate. This study investigated the relationship between total fertility rates in 187 countries and average latitude and average winter and summer temperatures. Total fertility rate in countries below 33° latitude is much higher (mean = 4.31) than it is in at higher latitudes (mean = 1.83). Fertility peaks at winter temperatures of approximately 21°C and falls for warmer winters and colder ones. Regression analysis found that the effects of latitude and winter temperature on fertility remained after controlling for societal variables including urbanization, population size and density, gross national product per capita, female literacy rates, and use of contraception. The data support the hypothesis that human reproduction is suppressed by short photoperiods and low temperatures. |