Geneticists in England and the United States clearly reversed their published remarks on the effects of race crossing between 1930 and 1950. The reversal occurred in two steps. First came the change in the 1930's from a condemnation of wide race crosses to an agnostic view. The second change, from the agnostic view to the belief that wide race crosses were at worst biologically harmless, took place during and shortly after World War II.

The entire reversal occurred in the light of little new compelling data from studies of actual human race crosses. The lack of new data is unsurprising. Few geneticists wished to initiate experiments that took three human generations to complete. And controlled race crosses are hard to arrange, even with government grants. What might be more surprising was the willingness of geneticists to make such positive statements about race crossing when they had so little reliable genetic evidence.

I interviewed or wrote to ten prominent geneticists who worked on human genetics between 1930 and 1950. Not one believed that new evidence on race crossing was the primary reason why geneticists changed their minds about the effects of race crossing. One plausible explanation, that the rise of "population thinking" (44) caused geneticists to change their minds, does not fit the evidence. Castle was no more of a "population" thinker than East, yet they differed radically in their conclusions about race crossing. What, then, did cause geneticists to change their minds?

Most important was the revulsion of educated people in the United States and England to Nazi race doctrines and their use in justifying extermination of Jews. Few geneticists wanted to argue, as had the Nazis, that biology showed race crossing was harmful. Instead, having witnessed the horrible toll, geneticists naturally wanted to argue that biology showed race crossing was at worst harmless. No racist nation could misuse that conclusion. And geneticists did revise their biology to fit their feelings of revulsion.

Geneticists' ideas about the related question of hereditary mental differences between races is perhaps undergoing a similar development to that seen earlier in their ideas about race crossing. In 1951, judging from the response to the Unesco second statement on race and comments in genetics literature, most geneticists agreed with Muller that races probably differed in significant average mental traits. By 1969, when Arthur Jensen advocated this view in his controversial article (45), most geneticists who spoke publicly on the issue had adopted an agnostic position. Knowledge of hereditary racial differences in IQ had scarcely changed since 1951, but society had changed considerably in racial attitudes. It will be interesting to see if during the next several decades geneticists will argue, on the basis of little additional evidence, that hereditary mental differences between races do not exist.

I am not condemning geneticists because social and political factors have influenced their scientific conclusions about race crossing and race differences. It is necessary and natural that changing social attitudes will influence areas of biology where little is known and the conclusions are possibly socially explosive. The real danger is not that biology changes with society, but that the public expects biology to provide the objective truth apart from social influences. Geneticists and the public should realize that the science of genetics is often closely intertwined with social attitudes and political considerations.