Wikipedia defines genetic determinism as “genes, along with environmental conditions, determine morphological and behavioral phenotypes”. Human behavior involves a number of aspects. Important among these is the tendency to develop a cultural identity. Cultural identities among humans are typically strongly associated with and aggressively defended against influences that can change or replace them. Thus most societies demonstrate prejudice, bias and ethnocentrism as means to safeguard the fidelity of their intrinsic cultures.
So why the big deal in safeguarding the integrity of ones culture? The clue to this question is in the Wikipedia definition cited above – genes. When one looks at cultural variation within human populations, genetic variations are also being evaluated. This is true because culture in many respects flags genetic distinctiveness. Especially true in the past, isolated groups of people developed a unique genetic signature and also a unique culture consisting of a unique language, religion and other social conventions. Tracking where these social traits go with time also tracks where the bundled genetic variation goes as well. Eventually, interbreeding with people of different cultures leads to genetic dilution and loss of both genetic distinctiveness and cultural traits. Xenophobic tendencies are focused on procreative fault lines dividing different gene pools.
Books and papers written by such noted scientists such as Richard Dawkins and others have helped to lay out the genetic rational for such base tendencies. It comes down to two important rules: (1) keep your “alien” genes from our women and (2) our genes are better than yours, so die! Genes are what they are by their unique sequence and they make their unique sequence known by their expression. Pairs of genes at a given locus compete for influencing phenotype. More dangerous to a gene’s dominance of phenotype is the process of chromosomal recombination through which parts of genes may be switched between paternal and maternally inherited chromosomes. If the recombination point is within the gene, it may loss its genetic distinctiveness forever. So interbreeding between ethnically distinct groups having distinct genetic variation is a threat to a gene pool’s dominance. This threat is mitigated when interbreeding occurs as a result of conquest. In this case the conquered males are reduced or eliminated and the women are inseminated with the victors sperm. Thus the victors’ genetic distinctiveness is preserved at the expense of that of the vanquished. As Conan the Barbarian answered to the question: what is best in life? “Crush your enemies, see them driven before you and to hear the lamentations of their women!”. Or put scientifically: Natural selection (the strong will survive to procreate and the weak will die). That’s human genetics at its most crude. Even in modern times, wars are fought ultimately to secure resources such as land and mineral wealth which always result in better procreative opportunities for the victors as shown by increasing populations. The enemy is always the baser society and we the better.
While prejudice, bias and ethnocentricity are cultural phenomena, there are strong biological imperatives that drive these behaviors. It is hoped that our logical minds can counteract these base impulses but it is fight with millions of years of evolution.