It's getting bigger all the time: Estimating the Flynn effect from secular brain mass increases in Britain and Germany

•Secular increases in brain mass are quantified in two countries (UK and Germany).•In the UK, brain mass increased by 52 g for males and 23 g for females over 80 years.•In Germany the increase was 73.16 g for males and 52.27 g for females over 99 years.•In terms of IQ, the UK gain = 0.19 and 0.08 points per decade for males and females.•In Germany, the IQ gain = 0.2 and 0.15 points per decade for males and females.

Secular increases in brain mass over nearly a century have been noted for both males and females in the UK and Germany. It has been argued that such trends may be associated with the Flynn effect. The IQ gain predicted on the basis of these trends is 0.19 and 0.08 points per decade for UK, and 0.2 and 0.15 points per decade for German males and females respectively, indicating a small contribution to the Fullscale IQ trends in these countries (2.95% of the German decadal gain and 12.73% of the UK gain). There is also a sex difference in the rates of brain mass gain in both countries, favoring males. Temporal correlations between the secular trend in UK brain mass and European Flynn effects on Fullscale IQ, Crystallized, Fluid and Spatial abilities reveal correlations ranging from 0.751 in the case of Fluid ability to 0.761 in the case of Crystallized ability. The brain mass increase may be an imperfect proxy for changes in specific neuroanatomical structures important for IQ gains. Its small contribution to these gains is also consistent with the influence of other contributing factors. Increasing brain mass is predicted by the life history model of the Flynn effect as it suggests increased somatic effort allocation into bioenergetically expensive cortical real estate facilitating the development of specialized cognitive abilities.