Numerous factors constrain population growth, most notably high mortality rates, low fecundity, mass emigration and low immigration. Environmental factors like disease and resource scarcity play a major role. Other factors are social, political and economic in nature.
Natural factors limit human population growth. Disease leads to high mortality for people of all ages. In some cases, infirmities also affect female fertility.
Scarcity of resources restricts growth. Populations grow in regions that support farming and have major water supplies. The lack of these features, and the presence of inclement weather, make it difficult to support large populations.
Other factors come from human affairs. Politics plays a role. Countries with stringent immigration policies limit the flow of migrant population growth. Some countries deliberately suppress population growth by establishing birth quotas, limiting families to a specific number of children.
Social and economic structures affect population growth. Farming-based societies often have high population growth because families have many children in order to tend to farm work. Parents in urban societies often have fewer children because children in such settings generally do not work, but still present an additional cost to the household. Moreover, the overall state of the economy is a factor. A poor economy leads to population reduction via emigration as people leave the region to look for work.
Culture is also an issue. Some cultures prize children and encourage childbirth even in urban settings. Other cultures think less highly of familial responsibility and lower fecundity by employing contraception.