I think the both rate thing is in reference to studies like those discussed here: https://www.lesswrong.com/posts/c4daDbQD398BmW5k5/on-car-seats-as-contraceptio

I think the both rate thing is in reference to studies like those discussed here: https://www.lesswrong.com/posts/c4daDbQD398BmW5k5/on-car-seats-as-contraception https://www.lesswrong.com/posts/c4daDbQD398BmW5k5/on-car-sea... The idea is that you can only fit two car seats in the backseat and upgrading to a larger car that could fit three is enough of an expense/hassle that it discourages having a third child