Repeating digits forwards is easy, and weakly predictive (.46) of general intelligence. Repeating digits backwards is harder, and more strongly predictive (.58) of general intelligence. Reliabilities are good if you give at least two trials for each digit string length. The task produces scores on a real, ratio scale, with a true zero, and thus is unusual in psychometrics in providing absolute results.
So, if there really is a Flynn effect, have digit spans have increased over the last century, particularly digits backwards, the better test of intelligence? “No” says Gilles Gignac from the bright blue skies of Perth, Australia. Not a glimmer of intellectual improvement since 1923. All this is as I had grimly expected. We shall all come to no good, just you mark my words.
Of course, perhaps digits backwards, demanding as they are, do not catch the full subtlety of Similarities or Vocabulary, or even Ravens Matrices. Let us dig around a little in this pre-publication paper, accepted by Intelligence.
In a careful approach, Gignac has gone back to the raw scores for longest digit spans forwards and backwards in the Wechsler intelligence test Digit Span subtest.
Gignac observes that if the Flynn effect is not acting on g and is not acting on short-term memory capacity, then it is hard to see that it is really acting on a broad range of fluid intelligence skills over time.
Turning the screw, Gignac points out that at the beginning of the century few people had to remember telephone numbers, but now we are inundated with long mobile phone numbers and login codes and the like, so there is a strong cultural reason for digit spans to have increased, but they have not.
He considers carefully the various explanations and details for the findings which might temper his conclusions, but in the end he clearly feels that it is very hard to explain how the Flynn effect, derived from standardised scores, can be real when it does not show up on actual raw scores of short-term memory.