Just out of curiousity, how many men in prison that allegedly have learning difficulties because of pot only use marijuana? And how many of these men didn't have learning difficulties before using pot? Your hypothesis is no good if you don't take the variables into account.
Funny the way it is that regular (responsible) users of marijuana say there is no lasting effect from marijuana, whereas people who oppose marijuana say there are detrimental lasting effects. Smoking marijuana can damage your lungs, but that is not caused by marijuana. That is caused by smoke. Other than that, I can't think of a single harmful lasting effect marijuana has on a person.
The one and only problem with marijuana is that it alters your state of mind temporarily. Anything else is just coincidental. If a person has learning difficulties, a bad attitude, does hard drugs, etc, it is not because of marijuana.