Intelligence 49 (2015) 23–24
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Intelligence
Do variable signal luminances and confounded stimuli contribute to slowing simple RT and cross study heterogeneity? A response to Parker (2014) Michael A. Woodley of Menie a,⁎, Jan te Nijenhuis b, Raegan Murphy c a b c
Center Leo Apostel, Vrije Universiteit Brussel, Belgium Work and Organizational Psychology, University of Amsterdam, The Netherlands School of Applied Psychology, University College Cork, Ireland
a r t i c l e
i n f o
Article history: Received 10 October 2014 Received in revised form 7 December 2014 Accepted 9 December 2014 Available online xxxx Keywords: Luminance Methods variance Heterogeneity Secular trend
a b s t r a c t Parker (2014) argues that contra Woodley, te Nijenhuis, and Murphy (2013), potential secular slowing of simple visual reaction time does not evidence decreasing intelligence, as there are other factors that covary with simple RT performance, not accounted for. These include variable cross-study signal luminance and conflated audio and visual signals. Scrutiny of these two proposed sources suggests that they are unlikely to contribute to either the heterogeneity or the direction of the apparent slowing trend. A sensitivity analysis furthermore indicates that Galton's sample could have been considerably slower than believed (213.85 ms) without the secular trend across the six US and UK samples examined in Woodley, te Nijenhuis, and Murphy (2014) becoming non-significant. © 2014 Elsevier Inc. All rights reserved.
Parker (2014) proposes that the argument put forward in Woodley, te Nijenhuis, and Murphy (2013); i.e. that the apparent secular slowing of simple visual reaction time across 115 years and 14 studies might evidence a diminution in general intelligence at the population level, is underdetermined. This is because several factors other than changing g, including signal luminances and conflated audio and visual stimuli, could potentially contribute to cross-study variance in reaction time performance. Parker argues that as these other factors were not accounted for adequately in the original study, their possible contribution to both the heterogeneity and direction of the trend detected in Woodley et al. (2013) cannot be ruled out. Although Parker (2014) discusses signal luminance and conflated audio and visual stimuli as only two possible (amongst
⁎ Corresponding author at: Center Leo Apostel, Vrije Universiteit Brussel, Belgium. E-mail address:
[email protected] (M.A. Woodley of Menie).
http://dx.doi.org/10.1016/j.intell.2014.12.003 0160-2896/© 2014 Elsevier Inc. All rights reserved.
several) sources of variance amongst the studies utilized in Woodley et al. (2013), discussion of these two sources will form the basis of the present comment. This is because Parker (2014) focuses his own commentary on these two sources. It will here be demonstrated that these two particular sources are, contra Parker (2014), unlikely to contribute variance to the trend detected in Woodley et al. (2013), and derivative work (Woodley, te Nijenhuis, and Murphy, 2014). Concerning between study heterogeneity, new findings reduce the plausibility of these sources having contributed to this. Woodley et al. (2014), in a follow-up study to their original 2013 study, found that matching studies more closely in terms of bio-cultural dimensions (i.e. using only the six studies from the US and UK), excluding a clear methodological outlier and also cleaning studies for methods variance stemming from stimulus onset delay, long and variable preparatory intervals and key pressure time (as per Dodonova & Dodonov, 2013) reduced heterogeneity in meta-regression substantially, yielding a very clear secular trend (p. 143–144).
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M.A. Woodley of Menie et al. / Intelligence 49 (2015) 23–24
For luminance differentials and signal conflation to have caused the trend, the stimuli presented in older studies should have been brighter and/or more conflated relative to more recent studies. Parker makes a compelling case that Geissler tubes generated noise in addition to light, thus they may have produced somewhat confounded signals. Therefore Thompson's (1903) 19th century study, which is known to have employed a Geissler tube as a visual signal source, may have yielded performance means for her students that were faster than would have been the case if a silent visual stimulus had been employed instead. The earliest version of Galton's apparatus utilized a mirror and a later version a disk of white paper (Johnson et al., 1985). We cannot know the range of luminance conditions associated with Galton's study, thus although Parker does not discuss this, luminance is a potential confound in the case of Galton's study also. There is evidence however that Galton's apparatus was unlikely to have generated confounding noise associated with its operation, as Galton (1908) reports that: “The tap that released the pendulum from a raised position made the required sound, — otherwise it made a quiet sight-signal, whichever was wished” (p. 248, italics added for emphasis). Comparing the means of Galton's and Thompson's male student samples with those of other male student means collected in the 19th century using other instruments might nonetheless give some indication as to their potential convergent validity. For example Donders (1868/1969) utilized a spark (p. 426) as the visual signal amongst his male student subjects, and found a mean simple RT of 188 ms. Wundt (1863) utilized a similar procedure and found a mean of 175 ms for his own male student sample. Galton's age 26+ ‘student-scholar’ simple RT mean appears to be similar to both of these findings at 185.5 ms (Johnson et al., 1985, p. 891; corrected upwards for the calibration error reported in Dodonova & Dodonov, 2013), as is the value for Thompson's (1903) male student cohort, i.e. 199 ms (all estimates fall below 200 ms). It may be that Donders, Galton, Thompson and Wundt all over-illuminated their subjects, or confounded their stimuli relative to more recent studies, however given the diversity of apparatuses and techniques used, coupled with the apparent congruence amongst these four results, it is also possible that as Kosinski (2013) noted with respect to 19th century college students and
scholars: “Reaction times may be getting slower, because we hardly ever see a Clemson freshman (or professor) who is that fast”. Even though it is not possible to estimate the cross-study methods variance due to signal luminance (Dodonova & Dodonov, 2013), it is nonetheless instructive to determine just how sensitive Woodley et al.'s (2014) most recent analysis of US and UK means is to imprecision in the value assigned to Galton's study, which of all of the studies has proven to be the most controversial. Reanalysing the six methods-variance-corrected US and UK studies indicates that the value associated with Galton's study could have been as high as 213.85 ms while still yielding a significant secular decline trend (b = .1575, 95% CI = .0001 to .315). This value is higher than the 207.5 ms estimate proposed by Dodonova and Dodonov based on their simulation, and is 26.85 ms slower than the value used in Woodley et al. (2014; 187 ms). While this sensitivity analysis does not directly address any particular source of method variance, it nonetheless gives some idea as to how robust the finding of slowing simple RT might be to potential biasing factors. References Donders, F. C. (1868/1969). On the speed of mental processes. Acta Psychologica, Attention and Performance II, 30, 412–431. Dodonova, Y. A., & Dodonov, Y. S. (2013). Is there any evidence of historical slowing of reaction time? No, unless we compare apples and oranges. Intelligence, 41, 674–687. Galton, F. (1908). Memories of my Life. London: Methuen. Johnson, R. C., McClearn, G., Yuen, S., Nagosha, C. T., Abern, F. M., & Cole, R. E. (1985). Galton's data a century later. American Psychologist, 40, 875–892. Kosinski, R. J. (2013). A Literature Review on Reaction Time. (Retrieved from) http://biae.clemson.edu/bpc/bp/lab/110/reaction.htm (08/10/14) Parker, S. (2014). Were the Victorians cleverer than us? Maybe, maybe not. Intelligence, 47, 1–2. Thompson, H. B. (1903). The Mental Traits of Sex. An Experimental Investigation of the Normal Mind in Men and Women. Chicago, IL: University of Chicago Press. Woodley, M. A., te Nijenhuis, J., & Murphy, R. (2013). Were the Victorians cleverer than us? A decline in general intelligence estimated from a metaanalysis of the slowing of simple reaction time. Intelligence, 41, 843–850. Woodley, M. A., te Nijenhuis, J., & Murphy, R. (2014). Is there a dysgenic secular trend towards slowing simple reaction time? Responding to a quartet of critical commentaries. Intelligence, 46, 131–147. Wundt, W. (1863). Vorlesungen über die Menschen- und Tierseele [Lectures on Human and Animal Psychology]. Germany, Leipzig: Voss.
