"Plenty of spanked children may grow into well-adjusted adults, but this is one reason why, for example, corporal punishment on average correlates with lower measures of cognitive ability, such as IQ. In these households, parents saw their role as providing shelter and basic support, but children’s development was encouraged to “spontaneously unfold” without sustained parental effort."

http://www.slate.com/articles/health_and_science/medical_examiner/2011/12/spanking_is_on_the_decline_why_.html

Justin