Oh, yeesh, not yet more junk science on IQ. First of all, digit span is exquisitely subject to training effects

http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordD...

(this is just one citation among many for this often-replicated result) and digit span is not related to important cognitive functions that sum up to "rationality" as distinct from IQ.

http://yalepress.yale.edu/yupbooks/book.asp?isbn=97803001238...

http://www.amazon.com/What-Intelligence-Tests-Miss-Psycholog...

But in general about articles in the popular press posted to Hacker News about IQ, what I say is that the obligatory link for any discussion of a report on a research result like that is the article by Peter Norvig, director of research at Google, on how to interpret scientific research.

http://norvig.com/experiment-design.html

Check each news story you read for how many of the important issues in interpreting research are NOT discussed in the story.