Percentage of random group of 1000 people who were spanked who think they "turned out alright": ~99%
If you polled those same people on whether they would judge that all the other people from that same group "turned out alright": much lower, probably around 50-60%
My point is that this is all very subjective. People who like spanking say they were spanked and that it made them a better person and point to other people they judge as good, who were also spanked, as further proof. But they ignore the undoubtedly numerous others who were spanked, and yet still turned out to be shitheads. And as confident as they are that spanking made them better people, it is impossible for them to know how they'd have turned out if alternative discipline techniques had been applied to them instead.
There are obviously a ton of people who were not spanked and grew up to be good people. So to me, the real question is not at all whether or not spanking is effective. The real question is, is it necessary? The fact that many people were not spanked, yet still grew into productive members of society, proves it not necessary. So my question is, why not focus our attention on developing effective parental discipline strategies and techniques that don't involve committing violence of any degree against children, since as stated, it's clearly not necessary?