Do variable signal luminances and confounded stimuli contribute to slowing simple RT and cross study heterogeneity? A response to Parker (2014)

Parker (2014) argues that contra Woodley, te Nijenhuis, and Murphy (2013), potential secular slowing of simple visual reaction time does not evidence decreasing intelligence, as there are other factors that covary with simple RT performance, not accounted for. These include variable cross-study signal luminance and conflated audio and visual signals. Scrutiny of these two proposed sources suggests that they are unlikely to contribute to either the heterogeneity or the direction of the apparent slowing trend. A sensitivity analysis furthermore indicates that Galtonʹs sample could have been considerably slower than believed (213.85 ms) without the secular trend across the six US and UK samples examined in Woodley, te Nijenhuis, and Murphy (2014) becoming non-significant.