Having reflected on the title of an earlier post which described the decline in intelligence since Victorian times in terms of approximately 15 IQ points (or one standard deviation):
And then thinking about the idea of using reaction times as a scale of general intelligence:
I realized that it would make more sense to most people if the decline in intelligence was expressed in terms of percentages.
Of course this is only approximate, but if the data from Silverman's paper is used the (median) average Victorian men's reaction time was 183ms (milliseconds), while the (mean) average Modern men's reaction time was 250 ms.
So the slowing from Victorian to Modern reaction times is 67ms
which is approx 37 percent of the Victorian reaction time of 183 ms; and 37 percent is more than one third.
So, in a phrase and using Victorian intelligence as the baseline; it would be reasonable to say that average intelligence had declined by more than one third since Victorian times.
And what does this 'one third' refer to?
MA Woodley talks of reaction times as a measure of core efficiency of the central nervous system - and this slowing of reaction times therefore suggests that our average thinking is biologically less efficient than the Victorians such that Victorian brains could perform an extra thirty seven percent more intelligent processing per unit time.