You may recall that I wrote with great enthusiasm about the wonders of digit span, describing it as a modest bombshell. It is a true measure: every additional digit you can remember is of equal unit size to the next, a scaled score with a true zero. Few psychometric measures have that property (a ratio scale level of measurement in SS Steven’s terms), so the results are particularly informative about real abilities, not just abilities in relation to the rubber ruler of normative samples. If there are any differences, including group differences, on such a basic test, then it is likely they are real.
Then last November Gilles Gignac dropped another bombshell. He found that if you looked at total digit span scores since 1923 there was not a glimmer of improvement in this very basic ability. This cast enormous doubt on the Flynn effect being a real effect, rather than an artefact of re-standardisation procedures. Gignac noted that digits forwards and backwards were in opposite directions, but not significantly so.
Later, I darkly suggested that another paper was in the offing, taking a different view of these data. Picture my pleasure when a barefoot messenger boy arrived at my humble cottage bringing a parchment missive with the unmistakeable seal of Woodley of Menie, the young inheritor of the great estate further North.
Young Woodley tells me that Gignac’s “substantial and impressive body of normative data on historical means of various measures of digit span covering the period from 1923 to 2008” reveals a hidden finding: the not-very-g-loaded digits forwards scores have gone up and the g-loaded digits backwards scores have gone down. This suggests that the fluffy forwards repetition task has benefitted from secular environmental gains, while the harder reversal task reveals the inner rotting core of a dysgenic society. Perturbed at these findings, I made sure that the messenger boy was kept outside, away from the under-maid, while I penned my reply.
Michael A.Woodley of Menie and Heitor B.F. Fernandes (2015) Do opposing secular trends on backwards and forwards digit span evidence the co-occurrence model? A comment on Gignac (2015). Intelligence 50 (2015) 125–130
Woodley and Fernandes opine: Gignac's (2015) observations are important as they add to the list of rarely considered cognitive measures the secular trends for which seem to defy the Flynn effect. These include inspection time, which, like digit span, seems to show no secular trends (Nettelbeck & Wilson, 2004), simple visual reaction time, which seems to show quite pronounced secular declines (Silverman, 2010; Woodley, Madison, & Charlton, 2014a; Woodley of Menie, te Nijenhuis, & Murphy, 2015; Woodley, te Nijenhuis, & Murphy, 2013b, 2014b) and also society-level indicators of cognitive capacity, such as per capita rates of macro-innovation and genius, which also indicate pronounced declines starting in the 19th century (Huebner, 2005; Murray, 2003; Simonton, 2013; Woodley, 2012; Woodley & Figueredo, 2013).
Woodley has proposed a co-occurrence model: losses in national IQ brought about by the higher fertility of the less intelligent exist at the same time as IQ gains brought about by more favourable social environments. “A rising tide raises all boats, but some are leaky”. More specifically, the dysgenic loss should be concentrated on g as dysgenic fertility and mutation load are biggest on more g-loaded subtests (Peach, Lyerly, & Reeve, 2014; Prokosch, Yeo & Miller, 2005; Woodley & Meisenberg, 2013a). Secular environmental IQ gains (i.e. Flynn effects) are concentrated on ability specific sources of performance variance (te Nijenhuis & van der Flier, 2013; Woodley, te Nijenhuis, Must, & Must, 2014c), thus narrow abilities should be increasing in parallel. Key to this is the observation that heritabilities rise with increasing g loading (Kan,Wicherts,Dolan,& van derMaas, 2013; Rushton & Jensen, 2010; te Nijenhuis, Kura, & Hur, 2014). Heritability may therefore mediate the effect of both dysgenic selection and related factors, such as mutation accumulation, and environmental improvements on IQ scores, concentrating the effects of the former on g and the latter on the specific variances of cognitive abilities.
Woodley and Fernandes have done a re-analysis of the forwards and backwards digit scores, using sample-size-weighted least squares regression. As befits his aristocratic origins, Woodley does not deign to make it clear what A and B stand for. In my role as interlocutor I can tell you that, for no particular reason, A = Backwards digits and B = Forwards digits. If only I had been consulted! Airliners have crashed due to such poor labelling, and operating table errors due to these baffling descriptions don’t bear thinking about.
In summary, digits forwards have increased somewhat, digits backwards have decreased. Woodley appears to have got another indicator of mild dysgenic trends.
Selecting the best scrap of paper I could find in the oaken desk, I acknowledged the kind communication of this intriguing finding, and bade Woodley of Menie a Happy Easter. I bestowed the same valediction on the messenger boy, plus the customary shilling, and sadly without the shilling, wish you the same.