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

Question

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?

In: 701

Answer 1

Simplest answer (also found with a fast google) is that male children are *slightly* more likely than female children so simply producing 2 kids biases you to a population with excess males. 2.1 children tips the odds to get enough female children in to the mix to maintain a population.