i read an article on the net saying that heavy marijuana use decreases one’s IQ by about 4 points which can be regained when the person stops smoking. but exactly how much is 4 points of IQ worth? a grade level? i know the importance of 4 points of IQ probably depends somewhat on one’s initial IQ anyways. for example, someone who only has an IQ of 74 would be considered slow if they lost the 4 points whereas someone with an IQ of 160 wouldn’t really miss that much if they went down 4 points.
Where ever you heard that is probably correct, but whether or not its a big enough difference is the point that you need to keep in mind. So while an average decrease of four is something you can measure, and its supported by statistics, it doesn’t mean terribly much. I don’t remember the exact numbers, but my guess is that you would probably show a greater drop in you IQ score, from being tired, or depressed, or any number of other random factors that would effect your test taking skills.
Of course, I’m relatively sure that abuse of marijuana, isn’t increasing your intelligence, I wouldn’t sweat 4 IQ points. I believe that studies have shown that repeated “heading” of soccer balls causes a bigger drop than that.
Also, while this isn’t really the place to get into this debate, the designers of the IQ test would be shocked at the way we use it today, its purpose was to select children in need of special education. So you were accurate in your comparison between people with IQ’s of 70 and 160, the test becomes pretty much insignificant to everything but a person’s ego once you reach scores above a hundred.
If you want to know EXACTLY what 4 IQ points says to the person interpreting the test well…
If you are Twenty years old, and have an average IQ, a loss of 4 points would mean that instead of having a mental age of 20, you would have a mental age of 19.2. Not something I would loose sleep over.
While I’m sure I’ve screwed something up, that will be corrected later, that is, to the best of my limited knowledge, how it works.
Mental age has not been the basis of IQ’s for many years now. Nowadays they standardize to a mean of 100 and a standard deviation of 15. So, for instance, 102 is the 55th percentile of IQ and 98 is the 45th percentile.
I used an IQ around 100 as an example, but it’s a bad example. That’s where it’s the most. In general, if your IQ is around 100 + 15z, then 4 IQ points corresponds to 10.6exp(-z[sup]2[/sup]/2) percentile points. So, for an IQ of around 100, z = 0, and you lose 10.6 percentile points, which is the example I gave. For an IQ of 125, z = 1.67 and you lose only 2.7 percentile points. For the extreme example of IQ = 160, z = 4, and you lose only 0.004 percentile points. It doesn’t seem like a lot, until you consider that you’ve more than doubled the number of people with higher IQs than you. 
I.Q. tests are neither accurate nor precise. This explains why “batboy” has an I.Q. of over 735.
I suspect that the “research” that showed that heavy marijuana usage decreased I.Q. by an average of 4 points is in fact total nonsense. It’s made to sound like it was a scientific experiment that went as follows: I.Q. tests were given to a large group of people. Then for several years they smoked a lot of marijuana. At the end of that time they were given I.Q. tests again. It was found that their scores had decreased by an average of 4 points.
Now what are the chances that an experiment like that would ever be done? In fact, this is probably what was done: A survey was done that identified a group of people who were willing to admit to being heavy marijuana users. They were given I.Q. tests. This group averaged a score of 96 on the tests. Since this is 4 points below 100, the average I.Q. score, this must mean that they lost 4 points by heavy use of marijuana. (It occurs to me that what may have been done was that the researchers may have contacted a bunch of people who were just arrested for marijuana use. They asked them if they would take an I.Q. test for this experiment, explaining that they would be paid for it. The users said, “Oh, wow, man. I can dig it.”)
This is ridiculous though. There’s no way to know if the heavy users had lower scores on average before they began using marijuana. There’s no way to know if the group of heavy users used for the experiment were typical of most heavy users. There are all sorts of other factors that may have contributed to their low scores. In fact, unless there were several hundred people in the group being tested, 4 points isn’t even statistically significant.