In outing Irving Fisher as a Social Darwinist, Bryan Caplan writes on how Fisher reconciled eugenics and economics. First, Caplan quotes Fisher:
The core of the problem of immigration is, however, one of race and eugenics. If we could leave out of account the question of race and eugenics I should, as an economist, be inclined to the view that unrestricted immigration, although injurious to some classes, is economically advantageous to a country as a whole, and still more to the world as a whole. But such a view would ignore the supremely important factors… Our problem is to make the most of this inheritance [of the 8,000 immigrants who arrived before 1741]. We can not do so if that racial stock is overwhelmed by the inferior stock which “assisted” immigration has recently brought. (“Impending Problems of Eugenics” )
Caplan considers that the two approaches diverge due to the inherent misanthropy of eugenics:
Economics doesn’t point to people and say, “Look what they can’t do.” Economics instead asks, “Well, what can they do?” If the answer is “something productive,” then the Law of the Comparative Advantage implies gains to trade. Economics, known for its hard-headed methods, culminates in an optimistic and humane conclusion: Regardless of their Darwinian “fitness,” the existence of people - even those well below average - makes the world a better place.
I would argue that the difference is more subtle than Caplan suggests, as Fisher and many other eugenicists were well aware of comparative advantage. Rather, they had different goals, particularly about the distribution of benefits, and they often came to a different conclusion about how someone’s negative externalities balance with the “something productive” that one can offer. At the limit, we regularly put people in prison for those externalities. The question is where the line gets drawn - Fisher would likely put more people into the negative externality basket than Caplan.
Further, eugenicists often understood that the poor had a comparative advantage (although I’d suggest they did not fully understand the full implications of this) when framing their policy preferences. Thomas Leonard writes:
Progressive economists, like their neoclassical critics, believed that binding minimum wages would cause job losses. However, the progressive economists also believed that the job loss induced by minimum wages was a social benefit, as it performed the eugenic service ridding the labor force of the “unemployable.” Sidney and Beatrice Webb (1897 , p. 785) put it plainly: “With regard to certain sections of the population [the “unemployable”], this unemployment is not a mark of social disease, but actually of social health.” “[O]f all ways of dealing with these unfortunate parasites,” Sidney Webb (1912, p. 992) opined in the Journal of Political Economy, “the most ruinous to the community is to allow them to unrestrainedly compete as wage earners.” A minimum wage was seen to operate eugenically through two channels: by deterring prospective immigrants (Henderson, 1900) and also by removing from employment the “unemployable,” who, thus identified, could be, for example, segregated in rural communities or sterilised. …
Worthy wage-earners, Seager (1913a, p. 12) argued, need protection from the “wearing competition of the casual worker and the drifter” and from the other “unemployable” who unfairly drag down the wages of more deserving workers (1913b, pp. 82–83). The minimum wage protects deserving workers from the competition of the unfit by making it illegal to work for less. Seager (1913a, p. 9) wrote: “The operation of the minimum wage requirement would merely extend the definition of defectives to embrace all individuals, who even after having received special training, remain incapable of adequate self-support.” Seager (p. 10) made clear what should happen to those who, even after remedial training, could not earn the legal minimum: “If we are to maintain a race that is to be made of up of capable, efficient and independent individuals and family groups we must courageously cut off lines of heredity that have been proved to be undesirable by isolation or sterilization . . . .”
These paragraphs also indicate the often missed difference between eugenics and Social Darwinism. Social Darwinists tend to be much more rosy about the effects of competition on the human race, while eugenicists would prefer to give it a push in their preferred direction.