I am interested by all these people stating that the memory loss reported could be attributable to many different causes. While this is true and no study is ever perfect, it is these findings are extremely provocative.
The reason you do a study with a large number of people is so that each group is considered 'Normal' - that is if there are x amount of people in group 1 who have had a poor memory from birth there will also be x amount of people in group 2 who suffer from similar problems. This has to do with the Law of Large Numbers and the Central Limit Theorem, which are the foundations of scientific research. Issues of confounding and blocking should always be addressed in this type of study in order to pass peer review
The idea is that by using large numbers of people who only differ in a single area (ie MDMA use), you can attribute differences to the use. The trouble is ofcoarse that statistically MDMA users are more likely to have used other drugs and/or come from different back grounds to non drug users. Nonetheless these findings are extremely interesting and not easily dissmissed.
The problem is that too many people on both sides of the argument allow their own emotions to shine through. This is just as noticable in the 'Oh no wont someone think of the Children!' camp as it is in the drug users world.
It is important to accept studies like this (so long as they pass peer review) and to include it in your decisions regarding your drug use.