Why is the replacement level for population considered 2.1 and not 2?

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I understand that many women will not have kids or will have only one kid, or that child mortality is involved but still a fertility rate of 2 means that ON AVERAGE every woman will have 2 kids. This means that every woman and man will be replaced, including the children that die young if the rate of 2 lasts (the newborn females will also have on average two kids). So why isn’t a fertility rate of 2 enough to replace the population?

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Anonymous 0 Comments

Easy math is that in non-replicating species such as humans it takes production of two offspring to merely replace the parents in the grander population perspective. A third production is required to affect a positive population increase at the individual (or couple, really) level, and it’s subsequent generation surviving until they they themselves reproduce effectively (again, with 2 children being replacement, one or none having reductionist influence, and 3 or more contributing to an overall increase in population).

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