How many countries are part of that OECD average? like, I'd like to see the individual data points that make up that average (I trust basically...no data that doesn't show individual data points).
In that chart, US looks like it could be the average of what could be extremely high tax rates and extremely low tax rates within that average. At least, one could just as easily make that argument without seeing the data. Or at the very least, you have a few massive outliers that are biasing that average way out of normal. (not saying that's the case...just why few people care to discuss averages if they are taking things seriously).
So yeah, obviously the overall trend is dropping, but what we do know is that a lot of developed nations suddenly came into being, like UAB and such during that trend, that have preposterously low, unsustainable self-destructive tax rates (because they think a commodity economy is sustainable, lol), that are likely influencing, if not the only reason, for that declining trend. I know that in much of Europe, the rate is much higher than in the US.