There are cold times of the year, and there are warm ones. Some seasons are dry while others, conversely, are rich in precipitation. The availability of food varies from season to season. In most species, attraction to the opposite sex manifests itself only during a certain period — mating season. Its timing is such that young are born when food is easier to find or when the probability of encountering predators is minimized.

Humans are one of the few animals that can reproduce all year round, but, in fact, we do not take full advantage of the opportunity given to us by nature. Studies of indigenous people living in Northern Canada and Papua New Guinea have shown that many more babies are born in certain months than in others. The majority of the Canadian Inuit respondents had birthdays in January–June. That is, they were conceived in April–September, when the environment is warmer, food is abundant, and there is less cause for concern.