Can intense multitasking improve fluid intelligence/working memory? The Dual N-Back task is the only task I'm aware of that has empirical support showing that it improves working memory. It appears to improve working memory through multitasking, although this multitasking must fit certain criteria, namely that you can't learn the task so well that you can eventually "automate" it without going into working memory.

Thus, I'm wondering whether other forms of multi-tasking such as playing Starcraft II (see my earlier thoughts) and driving with a cellphone can also improve working memory?

A possible follow-up question: if not, what differentiates the type of multitasking found in the Dual N-Back task from other types of multi-tasking?
In general, I'd hypothesise that "memory-training" programs will not lead to domain-general increases in fluid intelligence nor working memory.

As general background, you might want to check out the literature on expert memory.


Practice is very effective at improving performance on the practised task. Transfer is real and does exist, but it is often small in effect. 

Let's take the study by Ericsson, Chase, and Faloon (1980). After 230 hours of practice a participant, SF, was trained to increase their digit span (i.e., the sequence of random numbers they could recall) from 7 to 79 numbers. One might think that SF had increased his working memory. However, talk aloud protocols suggest that SF was using sophisticated domain specific mnemonic strategies (e.g., linking running times to random numbers). The skill did not generalise to other memory stimuli.
The expertise literature is filled with examples of experts seemingly defying the limits of human ability. Yet, such achievements tend to be domain specific and achieved through substantial practice. For an excellent review of the expertise literature, see Ericsson, Krampe, and Tesch-Romer (1993).

It is very difficult to modify very large domain general abilities.


Thus, my general advice is for people wanting to improve their working memory or fluid intelligence is to instead focus on what domain specific ability they want to improve and focus on practicing that. 

References


Ericsson, K. A., Chase, W. G., and Faloon, S. (1980). Acquisition of a
memory skill. Science, 208(4448):1181-1182. FREE PDF
Ericsson, K. A., Krampe, T., and Tesch-Romer, C. (1993). The role of
deliberate practice in the acquisition of expert performance.
Psychological Review, 100:363-406.