Offspring only count if the mother lives that long
- The idea
- Predict
- See it
- Why
- Check
The idea
“To get lifetime offspring, I add up the offspring per female at each age.”
A life table lists daughters per female at each age, so adding them up feels like the answer. Those numbers only apply to females who live that long.
The short version
The mistake: To get lifetime offspring, I add up the offspring per female at each age.
What's actually true: Each age's offspring count only for the females still alive at that age, so multiply by survival before you add.
- A life table follows a cohort, a group born together. Survivorship is the share of that group still alive at each age.
- Daughters per female applies only to females alive at that age. Multiply it by survivorship to get what that age adds per newborn.
- Add those products to get the net reproductive rate. Above 1, the population grows. Below 1, it shrinks.
On the exam: Multiply each age's offspring by the share of newborns alive at that age. Then add.