Fertility changes have an important role to play. Closely spaced births both weaken the mother, who has insufficient time to recover from the demands of the previous pregnancy before she is once again pregnant, and mean that children are weaned by the arrival of new baby earlier than they should be, placing them at greater risk of GI diseases such as diarrhoea, and subsequent nutritional weakening. The family age distribution is greater. The median age of infection for common childhood infections is consequently higher, by which time the child is stronger and more able to resist the infection. More children survive the infectionsso the mean age of the population increases, which gives the appearance of an increase in life life-expectancy.