From an NPR article here.
This guy has conducted two studies (published here and here) that suggest that not spanking leads to higher IQs. There have been other studies that demonstrate the connection between spanking and lower success rates as adults.
It's often difficult to make conclusions from statistical data like this; it could simply be the case that kids are poorer and dumber because their parents are poorer and dumber, and that poor and dumb people just happen to spank their kids more often. However, he claims he was able to adjust for socio-economic factors, indicating that the relationship between spanking and the child's IQs is causal.