As mentioned earlier, there are one or two physiological measures that have been found consistently to parallel IQ closely, and to be free from any possible training effect: reaction time and colour acuity.66 Simple reaction time has been measured in large populations since the middle of the nineteenth century, and has been found to be declining since the late nineteenth century.67 Using it as a measure suggests a real decline of 13.3 IQ points between 1884 and 2004.68 Colour acuity shows a similar trend.
