NEW YORK — A new analysis is challenging the idea that smoking marijuana during adolescence can lead to declines in intelligence.

Instead, the new study says, pot smoking may be merely a symptom of something else that’s really responsible for a brainpower effect seen in some previous research.

It’s not clear what that other factor is, said Joshua Isen, an author of the analysis. But an adolescent at risk for smoking pot “is probably going to show this IQ drop regardless of whether he or she is actually smoking marijuana,” said Isen, a lecturer in psychology at Loyola Marymount University in Los Angeles.

The study was released Monday by the Proceedings of the National Academy of Sciences. Some prior research has led to suggestions that the developing adolescent brain is vulnerable to harm from marijuana.

Studying the topic is difficult because children can’t ethically be randomly chosen to either take illicit drugs or abstain for years so that their outcomes can be compared. Scientists have to assess what people do on their own.

The researchers examined data that had been collected for two big U.S. studies of twins. They focused on 3,066 participants who were given a battery of intelligence tests at ages 9 to 12 – before they had used marijuana – and again at ages 17 to 20.

They tracked changes in the test scores and studied whether those trajectories were worse for marijuana users than for non-users. Most tests revealed no difference between the two groups, but users did fare more poorly than abstainers in tests of vocabulary and general knowledge.

However, among users, those who’d smoked more than 30 times or used it daily for more than six months didn’t do worse.