Whenever I would teach about stereotype dynamics, questions would inevitably arise about the legitimacy of racial categories and the expected results of cross-racial interactions. As anyone who was studied the subject quickly realises, the construct of "race" is very hard to pin down. In a social sense, race is loaded with meaning and consequence. In a biological sense, race is far more amorphous. But let us assume, for the sake of argument, that human racial categories are clearly separable and biologically meaningful. (Whether that is a worthy assumption can be left for another day.)

The largest chapter of human evolutionary history* depicts a hunter-gatherer species dwelling within subsistence groups, with periodic tribal coalitions. Even today, existing hunter-gatherer populations (e.g., the African !Kung, Brazilian Xingu, and Arctic Inuit peoples) tend to maintain strict rules about social interactions within and outside of one's group. So perhaps this is something deep within us, an elder prejudice from the days when we roamed the savannah. Most anthropological and biological evidence suggests that endogamy -- enforced marrying within cultural groups -- has been the norm during human history. Only very recently and grudgingly have these attitudes begun to shift.

Temporarily leaving aside concerns about preservation of cultural traditions, we might wonder if there is some biological imperative that would lead humanity to such mating practices. Some evolutionary psychologists have suggested that we choose to mate with similar others because this maximizes the likelihood that copies of our own genetic material will be passed down to future generations. Your children will inherit genetic characteristics either from you or from your mate; if you and your mate are near matches, gene-wise, then the desired effect is achieved regardless of which parent's characteristics are inherited. This is sometimes referred to as positive assortative mating. This would predict a biologically-driven preference for similar mates, and thus endogamous cultures should be the norm.

However, there is a problem. Experimental biologists have long known that extended periods of sexual inbreeding tend to exaggerate weaknesses within a population of organisms. Farmers and pet fanciers have known it longer still: inbred populations of pea plants and puppies alike are often physically smaller, immunodeficient, and show abnormal development. The reason is that the deleterious (harmful) genes within the inbred population are passed down and become more likely to be expressed with every new generation; this is sometimes referred to as inbreeding depression. For example, the majority of Dalmatian dogs are carriers for genes that result in deafness, and within this highly inbred population a significant minority (15-20%) are deaf in at least one ear. Similar phenomena can be observed within human populations (e.g., the high prevalence of sickle-cell anemics in sub-Saharan Africa; Tay Sachs disease in Ashkenazi Jews; various deformities and diseases within the Spanish Hapsburg royal line, culminating with the sad case of Charles II).

So if natural selection is working well -- "well" in this case meaning that the overall fitness of a population is increased -- then we should expect aversion to extended inbreeding periods. This may partially account for the commonality of incest taboos across the majority of human cultures across history. (Interestingly, however, siblings raised apart who meet later in life as apparent strangers may be quite attracted to each other -- another possible example of positive assortative mating.)

In essence, when very different genotypes are crossed, the inherited weaknesses from one parent will be balanced against inherited strengths from the other parent, and vice versa. The resultant offspring will tend to be more physically (and, if applicable, mentally) capable. This is sometimes referred to as hybrid vigor, heterosis, or outbreeding enhancement. For example, much success in experimental agriculture has resulted from better plant crops obtained by exploitation of hybrid gene lines.

It is therefore curious that eugenic movements have historically advocated for voluntary (or even enforced!) segregation of races when it comes to reproduction. Just as sauce for the goose is sauce for the gander, biological inheritance rules that govern "lower" plant and animal species should be considered relevant to the human condition as well. Although we cannot ethically conduct experiments with people in the same way we do with plants, there is a wealth of evidence to imply that heterosis would be ultimately beneficial for humankind. And this is to say nothing of the technological, aesthetic, and social benefits -- cultural evolutions -- that combined populations can create.

With such a stark disconnect between experimental biology and classical eugenics, between reality and historical policy, considerations of social and political factors must necessarily re-enter our consciousness. Why do mobs still rally around the cause of maintaining ethno-racial "purity", when we have seen the horrors that such causes can justify? Why do so many of us remain staunch tribalists?

I would love to hear thoughts from our experts in evolutionary psychology and other interested readers.

* Technically speaking, our biological evolution has not ceased and will not do so until our species goes extinct. Selection effects, both natural and artificial, continue to exert impact upon us. Any past tense (e.g., "when humans evolved, X, Y, and Z were common occurrences") must therefore be uttered with a grain of salt carefully positioned on one's tongue.