The secondary sex ratio represents the proportion of male and female births and is usually slightly skewed towards male births. Changes in the secondary sex ratio may be induced by a variety of ultimate (evolutionary) or proximate (physiological) causes. These causes tend to be linked to external factors such as natural disasters, armed conflicts, terrorism and/or important cultural and sports events. Natural disasters cause major destruction of housing, infrastructure and loss of lives. Such events are likely to be followed by a decrease in the secondary sex ratio. Stress-induced selection in utero against males is one of the widely discussed causes related to this phenomenon. Armed conflicts and terrorism commonly have a negative impact on society as well, however, an increase in the secondary sex ratio was often observed in the post-war periods. Important cultural and sports events, along with holiday celebrations, have a mostly positive impact on the social climate. These events induce a sense of community and are likely to be followed by an increase in the secondary sex ratio 9 months later, which suggests higher coital rates during such events.
