...there are two explanations for the relation of corporal punishment to lower IQ.
First, corporal punishment is extremely stressful and can become a chronic stressor for young children, who typically experience corporal punishment three or more times a week. For many it continues for years. The research found that the stress of corporal punishment shows up as an increase in post-traumatic stress symptoms such as being fearful that terrible things are about to happen and being easily startled. These symptoms are associated with lower IQ.
Second, a higher national level of economic development underlies both fewer parents using corporal punishment and a higher national IQ.Definitely food for thought.
And here's something else from biology, which represents a refreshing twist in looking at a question in a new way by completely rephrasing it. The article will take some thought to read, and many of you won't be interested; but those who are may find it intriguing.