The actual thing measured is visual reaction time, which has been shown to correlate with intelligence, as an "elementary measure of cognition." Apparently this has been measured with a high degree of accuracy since 1889. Who knew?
The article concludes with this statement which could explain some of the difference:
Previous research found in many Western countries women of higher intelligence had fewer children than women with lower intelligence.On the other hand, intelligent people tend to be healthier and live longer. Most of medicine as we know it has been developed in the last hundred years, enabling the less intelligent to live longer too.
