I'm not sure about this just because of the sheer number of people I know who were spanked and turned out fine. I actually don't remember a single person who was spanked (in my memory) that
didn't turn out fine.
I think there are better ways to punish a child, sure. But for a kid who just won't listen or in extreme cases, I believe a swat or two across the *** is a good way to get a point across-- you are the child, I am the adult, and you will behave. Period.
I do not think a spanking should ever be designed to actually hurt. Whenever I was spanked, it might have stung for a moment or two but it never hurt me. It was the embarrassment and humiliation of having been forcibly disciplined in front of my siblings, cousins, or friends that made me never want to be naughty again.
However, I do see how there could be a psychological effect. I distinctly remember one of my cousins being a holy brat, and my uncle (her father) turned her over his knee, placed one hand palm up over her butt, and smacked his hand with his other hand. He never even touched her, and everyone there (most of the extended family at the time) witnessed him not actually spanking her. But she cried for several minutes and insisted that her bum hurt. All in her head? Yes. Just the fact that she very firmly believed that she'd been hit when he had only hit his hand and not her at all speaks to the fact that there are psychological effects that people ought to be aware of.
It is my belief that spanking should never be a first option for punishment. It should be a last option, reserved for just absolutely unmanageable children or extreme circumstances. It should always be done out of caring and never out of anger, and never in a way that is designed to truly hurt. And it should be done with caution, always.
For goodness sake, I don't even spank my dogs very often. When they are bad I squirt them with a spray bottle. But in extreme cases? Sometimes it's just necessary. For example, in the first few weeks we had Pixel as a new puppy, she ran into the kitchen while I was cooking and had the oven open and very nearly jumped up onto the 350-degree hot oven door. I was terrified for her safety and slapped her away, hard. I'm sorry, but I would much rather have me slap her than let her burn herself badly. If I had a child in the same situation, yes, I would get them away from such danger by whatever means I felt was necessary at the time. Including hard physical force. If that makes me a bad person, well, so be it.