The master's thesis I edited in 2009 was a pilot study about corporal punishment and eating disorders, and turned up a correlation.
(For those currently saying "correlation doesn't equal causation," you're correct! The point of this study was to determine if there were higher rates of disordered eating in adults who'd experienced corporal punishment as kids. If the answer was no, then there's probably no statistical relationship. If the answer was yes--as it was--the research would then move on to "so is there causation here, or are there other factors?")
I don't bring this up because of the study itself. It was a small pilot study, I haven't seen any follow-up research, and the original isn't available online.
No, I bring this up because the author cited many other studies on the adverse effects of spanking to demonstrate why she thought the study was necessary in the first place.
THOSE STUDIES WENT BACK TO 1939.
ONE OF THEM WAS AN AGGREGATE METADATA STUDY.
If you're not familiar, that basically means its authors were studying studies. They took every single study on spanking they could find within a 70-year span and studied them all to see what patterns emerged. What they found was horrifying:
Every single study in that period, from the most rigorous right on down to "you can tell the authors wanted a spanking-is-good result," showed negative results from spanking. These were as diverse as increased risk of sexual abuse to depression to increased risk of substance abuse to poorer educational outcomes to greater likelihood of committing violent crimes as adults.
But that's not the horrifying part. If you're wondering how that could possibly not be the horrifying part, well...
NOT ONE STUDY SHOWED A POSITIVE TRADEOFF FOR THESE RISKS. NOT. ONE.
There was no "sports can cause injuries but can also improve self-esteem, personal fitness, and teach important disciplinary skills." There wasn't even a "homework has marginal benefits at most."
THERE WAS NOTHING. Multiple studies noted that while corporal punishment could produce the immediate appearance of improved behavior, what was actually happening is the kids were getting better at lying and hiding their undesirable behavior, and they were actually more likely to enter the juvenile detention system. The results were so overwhelmingly negative that even the biased studies deliberately designed to produce "it's good actually" results could not deliver those results.
In other words: in thousands of studies across ALMOST A CENTURY, we have never found a good reason to use corporal punishment. We haven't even found a mediocre one.