I have access to the paper, it's a shame that's still a problem with journals (the journal I am connected to, although I don't work in psychology, is open so the movement is slowly prevailing). So they argue that in the Victorian era people who were more successful had more kids and therefore we, presumably under the welfare state where unsuccessful people can still reproduce, are not as smart. To measure intelligence in those times they get use the literature on the correlation between reaction times (RT) and the g behind IQ. They seem to get around the Flynn effect by just concentrating on genetic heritable intelligence ("genetic g"), i.e. presumably they would therefore concede people are indeed smarter today but only due to non-genetic factors that are said to contribute to the Flynn effect like schooling and nutrition. That alone blows the link-bait headlines out of the water.

So the core of the paper is RT -> IQ correlation and then supporting their argument with measurements of RT over time. Let's examine them.

A) "Galton's measures indicated a simple visual RT mean of 183 milliseconds (ms) for a large sample of 2522 young adult males (aged between 18 and 30), along with a mean of 187 ms for a sample of 888 equivalently aged females."

B) "These means seem to be representative of the period as a 1911 review of various studies conducted in the last 19th and early 20th centuries (Ladd & Woodworth, 1911), which did not include Galton's measures, found an RT range of 151–200 ms (mean 192 ms), using different instrumentation to that employed by Galton (1889). Moreover, Silverman was also able to comprehensively rule out lack of socioeconomic diversity, as Galton's samples were diverse enough to be stratified into seven male and six female occupational groups (Johnson et al., 1985)... Twelve modern (post 1941) simple RT studies by contrast revealed considerably slower RTs for both males (mean 250 ms) and females (mean 277 ms) in a combined sample of 3836."

The key problem being "using different instrumentation to that employed by Galton", who they don't reveal, except by citation, was using some pendulum and string contraption to measure reaction time.[1] With a difference of only 67 milliseconds between the males of each group, and 90 milliseconds between the females you'd need to show an equivalence between Galton's method of measuring reaction time and later computer assisted measurement methods, which they don't appear to. They also make some adjustments to the times that they don't seem to detail fully, in favour of the effect they are looking for I think (I am just skimming the paper and doing a quick rebuttal here).

After the measurement problem there are other problems of control they don't deal with. They seem to control for occupations, but it's unclear what else they control for. For example one of the modern papers they use is from Australia where only people who responded to an invitation to take the study were included, and 20% or so of contemporary Australians are born overseas (often not from the UK) so it's not exactly directly comparable to 19th century UK. Then, as someone else has pointed out in this thread they don't defend against things that can change reaction time. It seems there can be a significant impact of diet on reaction time, in one caloric restriction study reaction time went down while word recall went up.[2] Circadian phase, hours awake and temperature all seem to also have huge effects.[3] So contrary to the headline not only are people today smarter and we are just talking about "genetic g" there is the potentially huge measurement error, diet, hours awake, circadian phase and temperature, so I think more than enough error to account for the 67-90 milliseconds they are talking about.

As we don't have even the beginnings of that kind of data from Galton, let alone even a reliable time measure, it's fairly pointless to try to use such a weak and contested proxy for intelligence in my opinion.

[1]http://galton.org/essays/1880-1889/galton-1889-rba-reaction-...

[2] page 18 http://www.uiteigenervaring.nl/Op_dieet/Tenslotte/Calorie_re...

[3] http://ajpregu.physiology.org/content/283/6/R1370.full.pdf+h...