An Australian study of 3796 14-year-olds has found that those who had been reported as having suffered abuse or neglect (7.9%) scored the equivalent of some three IQ points lower than those who had not been maltreated, after accounting for a large range of socioeconomic and other factors. Abuse and neglect were independent factors: those who suffered both (and 74% of those who suffered neglect also suffered abuse) were doubly affected.
Abused, neglected children have lower IQ in teens
Reference
Mills, R., Alati, R., O’Callaghan, M., Najman, J. M., Williams, G. M., Bor, W., & Strathearn, L. (2011). Child Abuse and Neglect and Cognitive Function at 14 Years of Age: Findings From a Birth Cohort. Pediatrics, 127(1), 4-10. Retrieved from http://pediatrics.aappublications.org/cgi/content/abstract/127/1/4