Age – sex structure means the ratio of females on males in different age groups of population. To express age – sex structure, population pyramids are used. 

Following factors are responsible for imbalance in age – sex structure.

1. Different birth rates: In the countries where high birth rate is found, the percentage of child age group (0 – 14 year) is always high, while the percentage of old age group (over 60 year) remains very low. On the other hand, in the developed countries of the world, due to low birth rate, the percentage of child group is low, but that of old group is high. Normally, more males take birth than of females in the world, this is why in the countries with high birth rate have more male children and so there is unfavourable sex ratio.

2. Difference between female and male birth rate: Due to lack of medical facilities in the developing countries, high infant death rate and high maternity death rate is found there. So in these regions, in age groups of 0 – 14 and 15 – 24, number of males is larger than the number of females. In the age group of above 65 years, female death rate is always less than the male death rate. Therefore, every country of the world has larger number of female population in this age group.

3. Migration: Generally, adults of the age group 15 – 59 always migrate for their living. In the regions from which the males migrate, number of females is always higher than male, but in the regions to where males go, the number of males is larger than number of females.

On the other hand, in developed countries of the world, female population migrating towards the cities from rural areas is found to be more. So, in the rural areas of developed countries, number of males is more than that of females.