New research found adolescents who abused marijuana and alcohol scored lower on a battery of intellectual aptitude tests than their drug-free peers, HealthDay News reported Oct. 19.

University of New Mexico (UNM) investigators asked 48 teens aged 12 to 18 to undergo a battery of tests to assess the effects of chronic substance abuse on their intellectual function. The tests measured a range of neuropsychological skills, including verbal reasoning, executive function, visuospatial ability, memory, and processing speed. Nineteen of the participants had a diagnosis of substance abuse or dependence, 14 were abstinent but had a parent who abused alcohol, and 15 were abstinent controls with no family history of alcohol abuse.

The researchers found that teens reporting the highest substance use had lower test scores across the board. In particular, teens who abused alcohol scored substantially lower on the test measuring executive function, while teens who abused pot scored lower on measures of memory performance.

Interestingly, teens who were abstinent but had a parent who abused alcohol were also affected, scoring lower on the test for visuospatial ability.

This is one of the reasons the results should be interpreted with caution, noted Ramani Durvasula, PhD, associate professor of psychology at California State University in Los Angeles. According to Durvasula, the authors failed to take into account socioeconomic and educational factors that influence intellectual development or to assess for problems at home.

“Kids who abuse drugs and alcohol are different from those who don't,” she said. “Let's face it, when kids are drinking 13 drinks a day (the study average), there's not a lot of parental supervision going on.”

Robert Thoma, PhD, associate professor of psychiatry at UNM and lead author of the study, agrees the results raise a “chicken and egg problem.”

“Which came first,” said Thoma. “The low executive function, which could lead to drinking more, or the heavy drinking, which leads to poor executive function?” Large longitudinal studies are needed to definitively answer to that question, he concluded.

The study was published online Oct. 19 in the journal Alcoholism: Clinical and Experimental Research.