If the countries are more similar, the results have higher precision. If less, then lower precision. Such things have been done. I remember a study discussed o

If the countries are more similar, the results have higher precision. If less, then lower precision. Such things have been done. I remember a study discussed on HN where they used the same neighbourhoods before/after a change of law. That wasn't porn though, I think it was strip clubs, and the result was something like "when a strip club opens in an area that had none before, the number of sexual assaults in the vicinity decreases by a few per cent".