These studies measured reaction times. There's only a negative .31 correlation between reaction time and intelligence test scores. Just chewing gum lowers your reaction times. http://en.wikipedia.org/wiki/Reaction_time#Mental_chronometr...

Actual intelligence scores have risen each decade: http://en.wikipedia.org/wiki/Flynn_effect

Also, the "We" in these studies is no longer just a narrow range of middle to upper class white males. If you look at sub-groups, most likely everybody is rising even when the overall average might be declining.

This is what happened with SAT scores since the 60s: "SAT scores don’t decline because students get dumber; they go down because more students take them." http://www.washingtonmonthly.com/college_guide/blog/average_...

So the question is, have reaction times gotten slower because of more people being tested, or other factors? I'd imagine in decades past, participating in a psych experiment was a more "special" occasion warranting heightened attention and performance than it is today.