A new study calls into question the results of a study published last year that concluded heavy marijuana use can permanently lower IQ by several points in teens. The new research suggests that the IQ drop may have been caused by factors related to economic class and home life, NBC News reports.

The new study used a computer simulation to demonstrate that the same effects on IQ could have been caused by socioeconomic factors, according to researcher Ole Rogeberg of the Frisch Center in Norway.

“The kinds of environments you are in do affect your IQ,” Rogeberg told NBC News. He noted that while a good education and challenging employment can increase intelligence, “if people are pushed out or decide to move out of these kinds of arenas, they will tend to see an IQ decline, and they will also be the type of people who tend to take up cannabis smoking during adolescence.” The findings are published in the Proceedings of the National Academy of Sciences.

The original study included data from 1,037 New Zealanders who were followed for more than three decades, starting at age 7.

The researchers found marijuana lowered IQ by as much as eight points for those who started using the drug in adolescence, and were heavy, lifelong users. The study found no effect on IQ in people who started using the drug as adults, which suggested marijuana was particularly harmful for developing brains.

Rogeberg noted his new findings do not mean marijuana is harmless. He recommended the original researchers should more thoroughly analyze their data to prove marijuana, and not other factors related to the marijuana-smoking lifestyle, can lower IQ.