Why isnt it just 2? What factors make up the 0.1?
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Future biologist here I’ll explain in a more scientific way and then in a more ELI5 way in a comment
When looking at population growth in ecology we just account for the members of the reproductive sex because it gives us a more precise outlook on the future of the population and the growth trend (females for humans), so the the factor to sustain a population is a little different than simply 2.1 babies/female.
I’ll skip the maths but basically the factor takes into account:
– Mortality rate of the specie per age range;
– Reproductive success (fertility) of a female that survives long enough to reach sexual maturity (how much females offspring they have on average each year);
– Life expectancy of a sexually mature female;
– Reproductive window (past a certain age the reproductive success is close to 0)
– The male to female at birth ratio
So the factor to sustain a population is 1, which essentially means that each female that can reproduce will in average have one female offspring (before dying) that will also have on average one female offspring that will reproduce
Simple, there is not the same number of men and women. Normally it is said there are 50% men and 50% women. It’s false. There are more men than women. Imagine there are 50.3% men and 49.7% women. This 49.7 needs to have more than 2 to compensate.
PD: I have to look for the exact figures, but this is the reason.
who is over here trying to sustain the population?
Some people die before they reproduce. Keeping the population at replacement level is not a matter of birthing 1 child for each parent. It’s a matter of 1 child for each parent reaching reproductive age to themselves then reproduce.
Think about parachutes that don’t open and bungee cords that break…basically, shit happens, and not everyone has kids…so we need a buffer.
If your batch of babies come in underweight, you get flogged, so you’ve make an extra baby limb or something to sure up the weight.
Because that’s what happens when you math numbers. Not everything is always going to come out even all the time. When you take an average you add all the numbers together then divide by the number of items you added.
It also helps account for people that might have 3, 4, or heck even 6 kids VS those who have zero or 1 kid.
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