Even to discuss eugenics in class terms is an enterprise of which many British eugenists earlier in the century would have disapproved, since they clung tenaciously to the view that the ideology they were propounding was sanctioned by science. Eugenics, they claimed, rested upon ‘laws of biology’, which no more admitted of subjective interpretation than did the ‘law of gravity’; those who denied these laws were guilty of the same sort of anti-scientific obscurantism which, a generation earlier, had led pious Christians into seeking to deny the validity of Darwinian biology. On the other hand, some eugenists did also devote much time and energy to debating the class implications of their creed and in the course of doing this they were to elaborate a highly idiosyncratic explanation of the British social system.

At the heart of eugenics was a fear about the likely consequences of differential class fertility. Eugenists interpreted the tendency of better-off groups to have smaller families than those beneath them in the social and economic scale to mean that the ‘superior’ stocks were dying out, while the ‘unfit’ continued to multiply; this demographic process, they argued, was producing progressive racial deterioration, alarming symptoms of which could already be seen. Eugenists were concerned, therefore, to stimulate the fertility of the better stocks (‘positive eugenics’) and to take whatever steps were feasible and politically acceptable to slow down the rate of reproduction at the bottom end of the social scale, steps which might include the placing of certain diseased and degenerate groups of people under custodial care so that they could be sexually segregated (‘negative eugenics’).