Here is a list of some objections to and evidence against the assertion that average Victorian IQ would have been measured at one SD higher than moderns - that is at a modern IQ of 115 or more.
My comments follow [in square brackets]
1. The decline of intelligence is too fast to be accounted for by known mechanisms related to differential reproductive success between the most and the least intelligent people.
[I agree, that mechanism only accounts for about half the rate of decline required to produce 1 SD slowing in simple reaction times, hence intelligence. Another mechanism, or more than one extra mechanism, is required. I favour the accumulation of deleterious (intelligence damaging) mutations generation upon generation, due to very low child mortality rates since 1800, compared with all previous times in history.]
2. A 1 SD decline in intelligence since Victorian times would lead to a collapse of high level intellectual activity such as the number of creative geniuses and the rate of major innovations...
[I agree - it would lead to collapse...]
but this collapse has not happened - therefore there cannot have been a 1 SD decline.
[But my interpretation is that collapse has happened: the number of creative geniuses has collapsed and so has the rate of major innovations. Unless we are fooled by hype, or the self-interested self-promotion of insiders, I think this collapse is very obvious indeed across the whole of Western culture. I was writing about this collapse for many years before I came across the evidence of reducing intelligence - but I was trying to explain it in other ways such as the decline in scientific motivation, honesty, institutional factors, modern fashions, bureaucratization, Leftism etc. But the data for intellectual collapse are solid: what is in dispute are the best explanations.]
3. Intelligence has been rising, not falling, in developed countries - as evidenced by the rising average IQ test scores - a phenomenon usually called The Flynn Effect.
[I agree that average IQ test scores rose through the twentieth century - but that this was a matter or rising test scores; meanwhile average intelligence was declining. In other words, test scores were subject to inflation - or more accurately stagflation: as when prices are rising but economic production is declining. IQ test scores were rising, but real intelligence was declining.]
4. The evidence of slowing simple reaction times is not valid, because measurements and sampling methods in Victorian times are too different from modern measurement and sampling methods.
[Michael A Woodley and I have argued that these micro-methodological quibbles are inappropriate and invalid - and I think we have refuted them.]
5. Simple reaction times are not a sufficiently accurate, or valid, measurement of intelligence. In fact the idea that reaction times measure intelligence is obvious nonsense, because the best fist fighters and athletes have the quickest reactions, so they would have to be the most intelligent people - but they aren't...
[Simple reaction times are nothing to do with what the general public thinks of as 'quick reactions', and nothing to do with athletics, sports, or that kind of thing. Since the mid 1800s it has been known that differences in simple reaction time - such as seeing a light flash and pressing a button, are correlated positively with differences in intelligence. The correlation is not very tight, there is a lot of scatter around the line, but there always is a correlation - and average sRT differences accurately predict measured intelligence differences between both individuals and groups such as class, sex and race. Nobody who knew the field disputed the robust correlation between sRT and IQ - and many of the main scholars (such as Jensen) have assumed that the reason for the correlation was causal - that sRT reflects speed of neural processing which is a fundamental aspect of general intelligence. It is dishonest scientific practice to overturn more than a century of good research just because the sRT results go in a direction that you find surprising.]
6. One SD slowing in sRT does not necessarily imply a 15 point reduction in IQ.
[I agree, because IQ is not a 'real' interval scale - which means that the difference in intelligence measured by 1 IQ point is not known and is presumably varied at different points in the scale. Reaction time is, however, an interval scale - measured in milliseconds. I have assumed that therefore sRT should take priority as the most valid scale and IQ should be calibrated against sRT. Therefore I argue that if sRT has slowed by about one SD then this should be understood to mean one SD decline in real intelligence. ]
7. An sRT slowing of about 70 milliseconds between the 1880s and nowadays may average at about 1 IQ point per decade, but this does not necessarily imply a linear rate of decline - the rate of change may vary.
[I agree. The actual rate of decline will depend on the main causes of decline. This is not known. Indeed, if I am correct that a generation upon accumulation of deleterious and intelligence-damaging gene mutations is an important factor - the way that this works is not known. My feeling or hunch is that this kind of effect would not be linear but that the incremental amount of damage would increase with each generation - perhaps exponentially or by some other accelerating rate. So that if there were 2 new deleterious mutations per generation, then 4 would be more than twice as harmful as 2; and 8 would be more than twice as harmful as 4 - and so on. So the rate of decline of intelligence (and slowing of sRT) over 150 years need not be linear - but I would guess it is accelerating.]
8. There is just not enough evidence. One historical study with not very many data points is not enough to overturn the consensus from the Flynn effect studies that intelligence is rising.
[Fair point - except that the current consensus is not very secure - since confidence that rising IQ test scores really means rising 'g' (general intelligence) has never been very high. But on the other hand, the sRT historical evidence of declining intelligence is too strong to ignore. The best response is to seek further methods of confirming the decline in intelligence using different data and methods. That is what Michael A Woodley and I are doing, as best we may - but it would be great to have other people also working on the problem.]