Okay - so simple reaction time data indicates that average general intelligence has declined by about one standard deviation (15 IQ points) since the late 1800s.
Should we believe it?
Well, here are a few considerations:
1. Simple reaction times are the most objective evidence we have concerning trends in general intelligence.
Therefore we should:
a. believe them, or
b. come-up with something better, or
c. show how these studies of reaction times are incompetent, or
d. assume that the studies of reaction times are dishonest, or
e. assume that this data is wrong on the basis that most data is wrong (and because we are under no obligation to believe any particular bit of data - quite the contrary, there must be good reasons for believing any specific proposition)
2. Any other data which can be brought to bear on the matter of declining intelligence is (I think) relatively 'soft', subjective and imprecise compared with reaction times - (things like rates of major innovation, rates of geniuses etc) so - although I see much of what seems to be consistent with a significant and relatively rapid decline in general intelligence - it is unlikely to convince someone who does not want to believe in the first place.
On the other hand, even if it does not support, does any of this long terms evidence of general intelligence contradict the thesis that it has declined?
I mean, is there any decent evidence that general intelligence has actually risen? (aside from the Flynn effect of increasing pen and paper IQ test scores - which is not relevant here since it is what is being explained, and therefore inadmissible as evidence).
Has, for instance, per capita performance improved since the late 1800s in any 'g' related and quantifiable human endeavour? I know this is tricky to answer, since there are shifts in the specialized activities of intellectuals (e.g. away from study of The Classics and towards some of the sciences, or medicine) - but is there is reasonably compelling evidence of this sort of improvement?
3. If is is decided that we should, after all, accept the best evidence as showing a significant decline in average intelligence since the late 1800s, then there is quite a lot of further work to be done on the mechanism - because, on the whole, present understanding of 'dysgenic' mechanisms in relation to intelligence is not adequate to explain the rapidity of this decline.
(I mean, that calculations based on differential reproduction and heredity of intelligence predict a much slower rate of decline than could lead to one SD reduction in 'g' in the space of just four or five generations.)
The accumulation of deleterious mutations with the relaxation of selection due to high rates of child mortality is my first best guess at the likely 'extra' mechanism (and Michael Woodley agrees)
but really this is just a best guess, and the detailed mechanisms of how this might work are unclear.
However, it could make interesting science in trying to find-out!