Men produce fewer sperm in summer than in winter, and taxi drivers produce fewer than men who do not drive.

Men who live in temperate climates tend to produce more sperm in winter as compared to the summer months. This is what partly explains the fewer birth rates witnessed in spring time, and less during the fall.

Studies across the years have ruled out less sexual activity during the summer as the cause of the low numbers in childbirth. Rather, the hotter months lead to a lower sperm count since high temperatures are known to inhibit sperm production. This is the same concept that finds taxi drivers produce less sperm than their male counterparts who don’t drive since their private parts (read balls) are continuously under exposure to heat for prolonged durations (so do couch potatoes, according to another study).
One of the most recent studies into the subject was in 2013 by researchers at the Ben-Gurion University of the Negev in Israel. This one established that the physical structure of sperm cells was healthiest in winter months, after testing 6,455 semen samples in a span of three years. However, despite decades of research, such inferences are not definitive and continue to remain an open topic.