Thomson discusses how intelligence is distributed between the various social or occupational levels of society. He explains the normal distribution of intelligence within the population, drawing parallels with the distribution of height.

Thomson refers to inquiries carried out by him in Northumberland in 1922, and inquiries carried out in the Isle of Wight in 1924, which demonstrated that individual differences outweigh social differences, and supported the notion that heredity plays more part than environment in determining intelligence. He addresses the problem of the differential birth rate, arguing that the best defence against this is mankind's differences, and these should be encouraged by equality in education, allowing secondary schools to be a mixture of abilities and talents opposed to segregation of intelligence.