And yet even if influence of car seat laws can be fully proven, it will be an influence on the wish for not having a third child, not a means of pulling through with that wish. Yet the title claims the opposite. Contraception and motivation to use contraception (or motivation to avoid need for contraception) are not the same thing. Note that I don't consider this inconsistency a mistake by the authors but as a playful deliberation instructing the reader in how to read the article.

So taken to its logical extreme - was China's "one child" policy, where having any more kids after the first one was heavily fined - was that fine a contraceptive? Or not? I'd argue that it was, because it prevents pregnancy, but not in the biological "stops the egg from being impregnated" sense.

It would make having a third child more costly, not alter people’s preferences. (If chocolate bars double in price, that doesn’t influence your wish for chocolate. It just makes it more expensive to satisfy.)

And yet people would willfully avoid buying chocolate. You are deliberately putting desires and intent in a single jar, but they are neither the same nor interchangeable. Desires can be conflicting amongst themselves (it's not even rare) while intent is the aggregate of desires, necessities, and, yes, outside power. Contraceptives are a means of achieving intent, not of altering it.