After suffering a stressful event such as a natural disaster, populations frequently experience a decline in the overall birth rate, accompanied by a rise in the proportion of female births. The explanation proposed is that female foetuses are more robust than male foetuses, so they are more likely to survive when the mother is under stress. The overall result is that fewer babies are born, but there is a higher proportion of females among those that do survive.

This is itself thought to be a clever natural mechanism to produce fewer male offspring when times are hard.

Males require more energy and resources to grow to maturity. Populations in trouble should invest more in females in order to produce enough babies for the next generation.

The following data (Fukuda et al, 1998) relates to the catastrophic Kobe earthquake in Japan in January 1995. Nine months later, in October 1995, there were 1971 males and 1961 females born in the affected region. The long-term ratio of males to females prior to the earthquake in this region was 0.516. Using the steps laid out in previous questions, test the evidence for a change in the sex ratio in October 1995, and interpret your answer.