Sir Ronald Fisher (1951) [3], the Professor of Genetics at Cambridge, who wrote a dissent stating that evidence and everyday experience showed that human groups differ profoundly “in their innate capacity for intellectual and emotional development” and that “this problem is being obscured by entirely well-intentioned efforts to minimize the real differences that exist.”
A reading of childhood rearing conditions of british working class households post-WWII can give a fair idea of the adverse circumstances operating against any such "well-intentioned efforts" at that time & place. Furthermore, such efforts likely failed to account for early (educational) developmental windows, combined nutritional differences, & given wide-spread racism & classism at the time. The efforts would be unlikely to counteract disparity in opportunity. Thus it is likely Fisher's (1951) assertion would not stand scrutiny today & may not be the best statement to be cited for Lynn's argument.
For a somewhat entertaining account of 1950s working class conditions & social mores, see Jennifer Worth's book (or tv series?) Call the Midwife.