From a psychological stand point I disagree that they know. I think it depends on the reason for the drug use. I think alot of people who use drugs as a form of self medication find that marijuana becomes less effective with time so they move on to the heavier stuff. Much as an alcoholic who starts out with beer moves on to hard liquor. But as far as experiementing I think you are more or less correct. So as far as marijuana being a gateway drug I think the best way to answer that is "it depends".
I disagree with this. I think that a person makes a conscious decision on what they want from drugs before they even do it- otherwise there would be no point to doing them.
Marijuana doesn't have a tolerance effect. Meaning, you don't need more to get the same high. Many people have smoked the same amount of marijuana since they first started.... and since the effective dose is so far removed from the lethal dose it's pointless to smoke yourself to oblivion not to mention impossible to do so.
In contrast a drug like heroine can be taken in the same amounts and have different effects on the body. For example if you take the same amount of heroine everyday for weeks but then take the same amount in a different location you can overdose. There is a strong tolerance buildup with a drug like heroine where a person feels they need more to satisfy their craving. That is part of the addiction- it's physical. That is not present in marijuana.