In one of the largest, most comprehensive studies on the topic, researchers found:
Source: http://news.utexas.edu/2016/04/25/ri...by-researchersThe more children are spanked, the more likely they are to defy their parents and to experience increased anti-social behavior, aggression, mental health problems and cognitive difficulties...
I'm sure there will be many anecdotal stories about how "I was spanked, and I turned out fine." Similarly, I'm sure many people could tell us how they used to smoke a pack of cigarettes a day, and yet they're currently completely healthy.
The science seems to be settled on this issue for the time being. Stop spanking your children.