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.
(2011). Child Abuse and Neglect and Cognitive Function at 14 Years of Age: Findings From a Birth Cohort.
Pediatrics. 127(1), 4 - 10.