The only kid I grew up with that was aggressive was pretty much the only kid on the block that wasn't spanked or disciplined.
I don't think you can pigeonhole everyone. Not all people are the same, not all people respond to things the the same, and that includes kids. I know that I have no tendency towards violence despite being spanked. I avoid violence at almost any cost within reason. I don't pretend that means it can never be harmful, but I needed it. I didn't listen. Perhaps the studies didn't exam all the variables. I know I was spanked and I'm fine with what happened to me. If my sister and her husband decided they were going to spank their kids (my niece and nephew), I wouldn't tell them that they're wrong to do it. But if he started coming home drunk and beating them, I'd step up and say something. If the kids are getting spanked for minor shit, I'd say something.
My sister was never spanked. She never needed it. I can guarantee you that a time out would have had no effect on me. I was like a Rottweiler. Call me a bad dog and send me to my cage, and I wouldn't care. My sister was like a Newfoundland Dog, simply raise your voice and she'd sit in the corner and cry until you came over and told her it was ok. My parents were perfectly capable of raising her without needing to resort to spanking. The idea that only bad parents have to do it is narrow minded, in my opinion. Not all kids are the same, not all kids respond to discipline the same, and regardless of ones own experience with their kids and their nieces and nephews, they may not have met a really terrible kid like I was. Had I not been disciplined, I would've turned out drastically different. Frankly, I'm glad I was born in the 70's. Had I been born in the 90's, I would've been disagnosed ADHD and drugged up instead of being disciplined and I never would've been able to do so many of the things I have.
DISCLAIMER: I'm not saying ADHD and things of that ilk aren't real things or that spankings can "cure" them or they can be taught away, I'm saying that as a child I exhibited all the classic symptoms and easily would've been MISdiagnosed as so many were in the 90s.