Prior to 1800 or so there were no large differences in per-capita GDP between nations, differences were perhaps on the order of 2-3 at most. As modern economic growth took hold in some nations and not in others, between-country inequality increased dramatically with differences in per-capita GDP between nations of up to a factor of 100. As more and more nations enter a modern economic growth phase–which now includes a very rapid catch-up phase–between-country inequality has started to decline. In the future we may return to much smaller differences in per-capita GDP between countries.
As MacAskill points out in Doing Good Better (review here see also here) this means that we live today in an unusual time when charity is very cheap. Today, for example, it’s possible to save a life for as little as $4000. As other nations become rich that will no longer be true. More generally, the average person in a developed country can do a lot of good today by giving up relatively little. As MacAskill writes:
Imagine a happy hour where you could either buy yourself a beer for five dollars or buy someone else a beer for five cents. If that were the case, we’d probably be pretty generous–next round’s on me! But that’s effectively the situation we’re in all the time. It’s like a 99-percent-off sale, or getting 10,000 percent extra free. It might be the most amazing deal you’ll see in you life.