How long do you want to live?

[--Pete]

Hi,

if the average number of children is n<2 per family ,then world population will decrease;if n>2 ,it will increase.[right][snapback]62369[/snapback][/right]

Not that simple. Consider a forty year life expectancy and each person engendering one replacement. In effect, there are only two generations alive at any one time and that gives you a certain fixed population. Now change only the life expectancy to, say, eighty years. Same replacement rate but the population will double because there are now four generations alive at the same time. What needs to be balanced to maintain a fixed population is the number of births versus deaths. If, as this thread implies, we are in for a vast increase in life expectancy, we will need a corresponding great decrease in childbirth to maintain a stable population. Otherwise, an increase in life expectancy by a factor of 17 will mean an increase in population by at least that much (and more, since we still have a worldwide population growth).

--Pete