yes, it should be. But there's 2 important points here. The first is that they have to be able to sing AND play their instrument just as well as they can do either on their own. It's not like most people literally can't do multitasking, it's just that they do both tasks worse than they would when single tasking when they try.
The 2nd is the point that that study only tested a limited type of multitasking, so we don't really know if every type is impossible for everyone, or if different people are capable of different types of multitasking.
For the record, i have never been able to sing and play an instrument at the same time at all. The only kind of multitasking i can do is walking and listening / talking. 
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Btw, i just want to point out that whenever you hear a statistic in science, it's generally similar to what i was talking about above. For instance, "doing X leads to a 20% higher risk of illness Y" doesn't necessarily mean that your personal risk of getting "illness Y" is 20% higher if you do X. It could mean that, of the population, 20% of the people are probably going to have some genetic feature that means that doing X too much will cause "illness Y" something like 100% of the time. Well, that isn't proven yet, and the reality is probably way more complex, but this is where science is heading, anyway. So, anyway, just be aware of this when reading scientific articles 
Last edited by IceCream (2012 April 01, 7:24 am)