Reaction time (e.g. pressing a button as fast as possible in response to a light or sound) correlates with general intelligence 'g' - faster reactions correlating with increased intelligence.
However, in principle reaction time would be expected to be less affected by socio-cultural factors than standard IQ tests.
Therefore long term trends in reaction time might be an objective and direct measure of true, underlying general intelligence compared with normal IQ tests.
In particular, reaction time trends might resolve the apparent paradox between
1. the long-term decline in inferred underlying general intelligence due to differentially greater fertility among those with differentially lower IQ (the 'dysgenic' argument);
2. and the long term increase in the results of normal IQ tests (the 'Flynn effect' - or more accurately 'Lynn-Flynn' effect, since the observation was first published by Richard Lynn).
The dysgenic argument is that g has declined over the past century and a half and the increase in IQ scores is superficial - in other words genetic intelligence has declined even while phenotypic intelligence has increased; while the Flynn effect argument is that phenotypic increase in measured IQ also reflects underlying an increase in underlying 'genetic' g.
In essence, dysgenics advocates agree with the Flynn-effect advocates that phentoypic IQ has increased over the twentieth century but infer that underlying, genotypic IQ has declined.
However, the dysgenics viewpoint has had no objective, directly measurable evidence that genotypic IQ has declined.
Dr Michael Woodley (http://publicationslist.org/M.A.Woodley) has pointed-out to me a paper by Irwin W Silverman of Bowling Green State University from 2010 which resolves this question, and provides convincing evidence to support the dysgenics argument.
Silverman IW. Simple reaction time: it is not what it used to be. American Journal of Psychology. 2010; 123: 39-50.
Abstract: This article calls attention to the large amount of evidence indicating that simple visual reaction time (RT) has increased. To show that RT has increased, the RTs obtained by young adults in 14 studies published from 1941 on were compared with the RTs obtained by young adults in a study conducted by Galton in the late 1800s. With one exception, the newer studies obtained RTs longer than those obtained by Galton. The possibility that these differences in results are due to faulty timing instruments is considered but deemed unlikely. Of several possible causes for longer RTs, two are regarded as tenable: that RT has been increased by the buildup of neurotoxins in the environment and by the increasing numbers of people in less than robust health who have survived into adulthood. The importance of standardizing tests of RT in order to enable more refined analyses of secular trends in RT is emphasized.
In a nutshell, Prof Silverman reviews data from Francis Galton between 1884 and 1893, extracted from a study of visual reaction times in 2,522 men and 302 women. The average reaction times were 183 milliseconds (ms) for men and 187 ms in women.
Silverman notes that in reviews of reaction time studies in 1911 (but not including Galton's work), it is clear that Galton's results were typical of the era - the range being from 151-200 milliseconds - median of 192 milliseconds.
By contrast, Silverman reviews twelve modern (post 1941) studies of visual reaction time (using a comparable methodology to Galton) - and the modern reaction times are very significantly longer - the total number of subjects was 3,836 - the mean reaction time was 250 milliseconds for men (SD 47) and for women was 277 ms (SD 31).
Looked at separately, in only one study, only for men, were Galton's average values contained within the 95 % confidence interval - in other words, in 11 of 12 studies and 19 of 20 comparisons - as well as the overall meta-analysis - the difference in reaction times reaches conventional levels of statistical significance.
Conclusion: Victorians had faster reactions, on average, than moderns.
Implication: Victorians were more intelligent, on average, than moderns.
It therefore seems that average reaction times have become slower over the past 100 years.
Since reaction times correlate with IQ then the measured decline in reaction times is consistent with a significant decline in general intelligence over the past century, as argued by the 'dysgenic' theorists.
Note added 30 April 2012.
I thought of this way of testing the dysgenic hypothesis using historical reaction time data in 2008, as shown by the following e-mail sent to Prof Ian Deary of Edinbugh University. However, at that time I could not find anything relevant in the online scientific literature, and gave-up looking.
E-mail: Monday 07.07.2008 timed at 09:51
I'm sorry to badger you when you are still catching up but I have had an idea for measuring dysgenic change by using reaction times which I am keen to follow-up.
Since reaction time correlates with IQ, and since reaction time is an old physological measurement, it is possible that there are representative data on national population reaction times over the past 100 or so years.
Because high IQ people have a fertility considerably below replacement level, my prediction would be that average reaction time in developed countries should have become longer, and that the standard deviation would have become smaller (due to selective loss of shorter reaction times).
But probably somebody has already done it (perhaps your group?)?
Or maybe reaction time correlates with 'phenotypic' (or measured) IQ (and therefore gets enhanced by the Flynn effect) rather than correlating with underlying 'genotypic' IQ? - I don't know.
If it hasn't already been done - do you know of any databases of reaction times (or somebody whom I might contact about this?
Best wishes, Yours, Bruce