Pioneering studies of IQ by G.H. Thomson and J.F. Duff – An example of established knowledge subsequently ‘hidden in plain sight’

SummaryPerhaps the earliest authoritative measurement of a social class gradient in IQ, with a stratification of occupations among the parents of children with different IQs, is seen in two fascinating papers published in 1923 and 1929 in the British Journal of Psychology. The authors were GH Thomson and JF Duff (both of whom were later knighted) and the papers’ main findings were confirmed by later researchers. Results of an intelligence test administered to 13419 children aged 11–12 were analyzed according to parent’s occupation. The average children’s IQ at extremes of social class among their parents included clergymen-121, teachers-116 and bankers and managers-112 at the upper end; while at the lower end there were ‘cripples and invalids’-94, cattlemen-93, hawkers and chimneysweeps-91, and the ‘insane, criminal’-88. More than 100 specific categories of parental occupations were then combined into 13 social classes, with their children’s average IQ as follows: Professional-112; Managers-110; Higher Commercial-109; Army, Navy, Police, Postmen-106; Shopkeeping-105; Engineers [ie. apprenticed craftsmen, such as mining engineers]-103; Foremen-103; Building trades-102; Metal workers, shipbuilders-101; Miscellaneous industrial workers-101; Miners and quarrymen-98; Agriculture-98; Labourers-96. A follow-up study compared an ‘intelligent’ group (IQ 136 plus) with a matched IQ 95–105 ‘control’ group. IQ testing at age 11–12 was predictive of teacher’s reports of higher levels of intelligence and health at age 16; and better performance in official examinations. The occupations of fathers, grandfathers and uncles were consistent with occupation being indicative of ‘an inherited quality’ (i.e. IQ) and there was regression from parents to grandparents and uncles among the ’intelligent’ but not among controls. Other findings included a wider variance in intelligence among boys than girls, and descriptions of the predictive value of IQ in estimating future education, examinations and health. Although the distribution, heredity and predictive value of childhood IQ measurements was once quite widely understood, for the last few decades IQ research has been regarded as morally-suspect and IQ scientists subjected to vilification, persecution and sanctions. Ignorance and misunderstanding of IQ is the norm among intellectual elites in schools, universities, the media, politics and public administration. Consequently IQ research is actively-shunned, and has near-zero influence on public policies. Since this area of science has so been comprehensively ‘disappeared’ from public consciousness as a result of socio-political pressure; it seems probable that other similarly solid and vital domains of scientific knowledge may also be ‘hidden in plain sight’.