Has GDP gone up or down over time? It's gone way up, so even if the percentage was constant, the amount of money collected has increased significantly. This is what I mean by a clear trend towards "not-less". Despite this increase, I don't think we're seeing proportional returns, and I'm very doubtful that increasing it even further will help anything. If the root problem is mismanagement/corruption/inefficiencies/pointless wars, throwing more money at it isn't going to fix anything, while taking money away could.