Put the pipe down, kid. Teenagers who frequently use marijuana may be doing irreversible damage to their intelligence, attention span, and memory, according to a major study spanning four decades. Researchers examined more than 1,000 New Zealand residents, and found that young people who began smoking in their formative years had remarkably lower IQs than subjects who either didn't smoke or who only started using pot when they were adults. Here, a concise guide to the findings:

How did the researchers conduct the study?
The study, published in the Proceedings of the National Academy of Sciences, followed 1,037 people from birth to age 38. Subjects had their intelligence quotient, or IQ, tested at ages 13 and 38, and were monitored for marijuana use throughout their lives. Around 5 percent of the people tested were considered to be dependent as teens, in that they used cannabis at least once a week.

What did researchers find?
Subjects who began smoking pot as teenagers saw an eight-point IQ drop-off when they were tested again later in life. Interestingly, people who starting using pot as adults showed no decrease in IQ, while people who never used it at all actually saw their intelligence quotients increase.

Is eight IQ points a lot?
While eight IQ points on a scale where the mean is 100 "may not sound a lot," a drop from 100 to 92, for example, "represents a move from the 50th to the 29th percentile," says Sarah Boseley at Britain's The Guardian. Other studies have proven that higher IQs typically correlate with "higher education and income, better health, and a longer life."

So what does this mean?
The brain is "especially vulnerable" during the teen years up through the early 20s, and frequent marijuana use during adolescence could prove detrimental, says study lead author Madeline H. Meier at Duke University. This doesn't mean you can never smoke pot, "but if you do it during this critical period of development, you'll get these long-term negative changes," Stacy Gruber, an assistant professor of psychology at Harvard University who wasn't involved in the study, tells Bloomberg Businessweek. "In almost every case, the subjects who started as adults don't have declines. Those who started as teenagers do." 

Sources: Bloomberg Businessweek, CNN, The Guardian, The New York Times