The magical numbers 7 and 4 are resistant to the Flynn effect: No evidence for increases in forward or backward recall across 85 years of data

A substantial amount of empirical research suggests that cognitive ability test scores are increasing by approximately three IQ points per decade. The effect, referred to as the Flynn effect, has been found to be more substantial on measures of fluid intelligence, a construct known to be substantially correlated with memory span. Miller (1956) suggested that the typical short-term memory capacity (STMC) of an adult is seven, plus or minus two objects. Cowan (2005) suggested that the typical working memory capacity (WMC) of an adult is four, plus or minus one object. However, the possibility that both STMC and WMC test scores may be increasing across time, in line with the Flynn effect, does not appear to have been tested comprehensively yet. Based on Digit Span Forward (DSF) and Digit Span Backward (DSB) adult test scores across 85 years of data (respective Ns of 7,077 and 6,841), the mean adult verbal STMC was estimated at 6.56 (± 2.39), and the mean adult verbal WMC was estimated at 4.88 (± 2.58). No increasing trend in the STMC or WMC test scores was observed from 1923 to 2008, suggesting that these two cognitive processes are unaffected by the Flynn effect. Consequently, if the Flynn effect is occurring, it would appear to be a phenomenon that is completely independent of STMC and WMC, which may be surprising, given the close correspondence between WMC and fluid intelligence.