I don't spank, but up until 30 or 40 years ago, spanking was a perfectly acceptable form of punishment, and probably most kids were spanked at one time or another. And it wasn't like society was significantly more aggressive than it is now. So I don't know how you can make that sweeping generalization -- at least post links to some studies.
Here is one study that found that the link between spanking and aggression is greatly determined by the context in which the spanking occurs: https://files.eric.ed.gov/fulltext/ED415019.pdf