Very cold areas with a short growing season.
Temperate regions with no extremes of temperature.
Areas with constantly high temperatures and little rainfall.
The death rate is higher than the birth rate.
The birth rate is higher than the death rate.
Laws limiting family size. e.g. China's one child policy.
Greater education of females.
Incentives to limit family size.
Maternity & paternity pay.
Greying of the population.
Ageing of the population.
Retirement of the population.
Someone forced to move because of war.
Someone going on an extended holiday.
Someone moving for a higher standard of living.