Others have pointed out, too, that genetic and evolutionary studies may yield unpleasant results.
I think that many purely secular liberals who pride themselves as being above the "superstitious" religious types would be far happier with the concept of all man being equal, endowed by THEIR CREATOR with certain inalienable rights, than the ultimate possible conclusions of the purely scientific, genetic approach.
The line "one nation under God" and the concept that our rights are innate and dervie from God and not the state work to make sure (as much as possible) that THE STATE is not the sole authority.
dervie = derive
So would many literalist Christians and other theists, whose world view rests heavily on the assumption that behavioral patterns which are viewed as criminal and/or immoral are of entirely spiritual origin, and can be completely remedied by believing the right things.
Both atheistic or agnostic liberals who refuse to admit that anyone is inherently better than anyone else, and religious people who refuse to admit that not all problems can be solved via religious belief and/or practice, are very stuck on the notion that social ills can only be remedied by widespread adoption of THEIR particular set of rules and beliefs. Behavioral/cognitive genetics flies in the face of this concept, hence both groups fight it energetically.