According to an international study done by William Bainbridge, [[atheism]] is frequent among people whose interpersonal social obligations are weak and is also linked to lower fertility rates in advanced industrial nations (See also: [[Atheism and social/interpersonal intelligence|Atheism and social skills]]).<ref>Bainbridge, William (2005). "[http://www.religjournal.com/pdf/ijrr01002.pdf Atheism]" (PDF). Interdisciplinary Journal of Research on Religion. 1 (Article 2): 1–26.</ref> See also: [[Atheism, women and children]]
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Replacement fertility is the level of fertility that is required to sustain a population without any external inputs (see also: [[Population crash]]).
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In human populations, replacement fertility is 2.1 children born per woman in a given population. Worldwide, the fertility rate has fallen from an average of 5 births per woman in 1950 to 2.3 births per woman in 2021.<ref>[https://www.unfpa.org/swp2023/too-few#:~:text=Since%201950%2C%20global%20average%20lifespans,births%20per%20woman%20in%202021. The Problem With Too Few], United Nations</ref>
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Caspar Melville wrote in ''The New [[Secular humanism|Humanist]]'': "Firstly [[Secular left|secular liberalism]] is individualistic, and therefore it goes hand in hand with delayed child bearing and lower fertility rates.<ref>[https://newhumanist.org.uk/articles/2267/battle-of-the-babies Battle of the Babies] by Caspar Melville, The New Humanist</ref>