Spanking and Mental Health

New research discussed in the Chicago Tribune reveals that spanking in childhood is linked to a higher incidence of mental illness in adulthood. The article, entitled "Spanking Linked to Increased Risk of Mental Health Problems" reveals that spanking and harsh physical punishment in childhood are linked to higher rates of mania, alcohol or drug abuse/dependency, and personality disorders. What are other factors that could be linked here? How would you explain the relationship between spanking and SES that seems contrary to past research?