Income and fertility

Income and fertility are both ways of measuring how much people have in life. Income is how much money someone has to spend, and fertility is how many children a person can have. Usually, when people have more money (higher income), they can afford to have more children (higher fertility). That's because having children costs money - for things like food, clothes, healthcare, and schooling. So people with more money tend to have more children, and people with less money tend to have fewer children.