As it turns out, the numbers depend on where in the world you live. According to one literature review (PDF), in southern and tropical climates, births tend to decrease in the spring, indicating that conception was less common during the previous summer. Researchers postulate that deterioration of sperm quality during the hot summer months may depress fertilization success. Other research shows a decrease in testosterone levels—and therefore possibly sexual desire—in men during the same period. People living in cooler northern climes demonstrate the opposite pattern, with peak birth rates occurring in the spring—nine months after summertime.