content: The assumption in your first paragraph is that things have to continue once they start.    That is false. Look at nature - when food is scarce, the animal populations shrink, many times to much smaller levels.  When the food returns the populations of the animals bounce back completely.  How are humans different? Birthrate does not equate to population - as you are inferring.  Birthrate is the amount of children per woman.  The population includes old and young.  Birthrate is not a biological condition - it is a social/cultural/economic construct and can change every 9 months.