With the recent confirmation from reaction time measurements that average intelligence has objectively and significantly declined from Victorian times until now,
the next step is to start taking this fact into account, and unpacking its implications...
First, how much has reaction time/ intelligence declined?
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 0.187 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.
We do not have a standard deviation (measure of scatter) for the Victorian data - so we need to compare (looking at men) a (mean) average modern reaction time of 250 milliseconds (SD 47) with a (median) average Victorian RT of 183.
This implies that average (and being conservative in my interpretation) Victorian reaction times were more than one standard deviation faster than modern RTs; or, that the average Victorian would be placed comfortably in the top 15 percent of the modern population - probably higher.
If we assume that reaction time is a valid measure of general intelligence, in other words that RT has a linear correlation with g - then this would mean that the average Victorian Englishman had a modern IQ of greater than 115.
Does this degree of difference in IQ make for a significant difference in performance?
Well, yes - it certainly does.
The difference between the modern IQ standardized at 100 and the Victorian IQ of 115 plus would be somewhat greater than:
1. The difference between an unselective 'comprehensive' school which had an average population, and a highly selective 'grammar school'
2. A mainstream US state university and an Ivy League college
3. The cognitive ability of high school teachers compared with doctors
4. The measured IQ difference between Europeans and Ashkenazi Jews (as described by Cochran and Harpending, or Richard Lynn)
These levels of IQ difference would unpack to make very substantial differences in the attainment of high level intellectual activities: just as, for example the proportion of successful scientists, writers, lawyers and chief executives that are produced is very different for an Ivy League college than for a big State University.
So, while there would be an overlap of something like 10-15 percent - and therefore many individual exceptions - the difference in intelligence between moderns and Victorians would readily be observable at the group level: and the decline would be obvious - at least to the Victorians!