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In humans

Assortative mating in humans has been widely observed and studied, and can be broken down into two types of human assortative mating. These are genetic assortative mating, or assortative mating with mate choice based on genetic type and phenotypical expression, and social assortative mating, or assortative mating with mate choice based on social, cultural, and other societal factors.

Genetic assortative mating is well studied and documented. In 1903 Pearson and colleagues reported strong correlations in height, span of arms, and the length of the left forearm between husband and wife in 1000 couples. Assortative mating with regards to appearance does not end there. Males prefer female faces that resemble their own when provided images of three women, with one image modified to resemble their own. However, the same result does not apply to females selecting male faces.

Assortative mating based on genomic similarities plays a role in human marriages in the United States. Spouses are more genetically similar than two randomly chosen individuals. The probability of marriage increases by roughly 15% for every 1-SD increase in genetic similarity. However, some researchers argue that this assortative mating is caused purely by population stratification (the fact that people are more likely to marry within ethnic subgroups such as Swedish-Americans).

At the same time, individuals display disassortative mating for genes in the major histocompatibility complex region on chromosome 6. Individuals feel more attracted to odors of individuals who are genetically different in this region. This promotes MHC heterozygosity in the children, making them less vulnerable to pathogens. Apart from humans, disassortative mating with regards to the MHC coding region has been widely studied in mice, and has also been reported to occur in fish.

In addition to genetic assortative mating, humans also demonstrate patterns of assortative mating based on sociological factors as well. Sociological assortative mating is typically broken down into three categories, mate choice based on socio-economic status, mate choice based on racial or ethnic background, and mate choice based on religious beliefs.

Assortative mating based on socio-economic status is the most broad of these general categories. It includes the tendency of humans to mate within their socio-economic peers, that is, those with similar social standing, job prestige, educational attainment and economic background as themselves. This type of assortative mating includes the growing tendency in recent years for humans to marry people more like them in educational attainment or earned income. This applies across all socio-economic statuses, both rich and poor, in that the rich and well educated tend to marry among themselves, as do the poor and under educated. This is best observed in the fact that, in the United States, matches among those with similar educational attainment were more common than they would have been if couples had matched randomly.

Another form of sociological assortative mating is assortative mating based on racial and ethnic background. Mentioned above in the context of the genetically similar preferring to mate with one another, this form of assortative mating can take many varied and complicated forms. While the tendency mentioned above does exist, and people do tend to marry those genetically similar to themselves, especially if within the same racial or ethnic group, this trend can change in various ways. It is common, for example, for the barriers to intermarriage with the general population experience by a minority population to decrease as the numbers of the minority population increase. This assimilation reduces the prevalence of this form of assortative mating. However, growth of a minority population does not necessarily lead to decreased barriers to intermarriage. This can be seen in the sharp increase in the non-white Hispanic population of the United States in the 1990's and 2000's that correlated with a sharp decrease in the percentage of non-white Hispanics intermarrying with the general population.

Religious assortative mating is the tendency of individuals to marry within their own religious group. This tendency prevalent and observable, and changes according to three main factors. The first of these is the proportion of available spouses in the area who already follow the same religion as the person searching for a mate. Areas where religious beliefs are already similar for most people will always have high degrees of religious inbreeding. The second is the social distance between the intermarrying religious groups, or the physical proximity and social interactivity of these groups. Finally, the third factor is the personal views one holds towards marrying outside of a religion. Those who greatly value adherence to religious tradition may be more likely to be averse to marrying across religious lines.

Effects

Assortative mating has reproductive consequences. Positive assortative mating increases genetic relatedness within a family, whereas negative assortative mating accomplishes the opposite effect. Either strategy may be employed by the individuals of a species depending upon which strategy maximizes fitness and enables the individuals to maximally pass on their genes to the next generation. For instance, in the case of eastern bluebirds, assortative mating for territorial aggression increases the probability of the parents obtaining and securing a nest site for their offspring. This in turn increases the likelihood of survival of the offspring and consequently fitness of the individuals. In birds whose coloration represents well being and fecundity of the bird, positive assortative mating for color increases the chances of genes being passed on and of the offspring being in good condition. Also, positive assortative mating for behavioral traits allows for more efficient communication between the individuals and they can cooperate better to raise their offspring.

On the other hand, mating between individuals of genotypes that are too similar allows for the accumulation of harmful recessive alleles, which can decrease fitness. Such mating between genetically similar individuals is termed inbreeding which can result in the emergence of autosomal recessive diseases. Moreover, assortative mating for aggression in birds can lead to inadequate parental care. An alternate strategy can be disassortative mating, in which one individual is aggressive and guards the nest site while the other individual is more nurturing and fosters the young. This division of labor increases the chances of survival of the offspring. A classic example of this is in the case of the white-throated sparrow (Zonotrichia albicollis). This bird exhibits two color morphs – white striped and tan striped. In both genders, the white striped birds are more aggressive and territorial whereas tan striped birds are more engaged in providing parental care to their offspring. Therefore, disassortative mating in these birds allows for an efficient division of labor in terms of raising and protecting their offspring.

Positive assortative mating is a key element leading to reproductive isolation within a species, which in turn may result speciation in sympatry over time. Sympatric speciation is defined as the evolution of a new species without geographical isolation. Speciation from assortative mating has occurred in the Middle East blind mole rat, cicadas, and the European corn borer.

Humans display these genetic results of assortative mating too, just like other animals. What makes humans unique, however, is the human tendency towards seeking mates that are not only similar to us in genetics and in appearances, but those who are similar to us economically, socially, educationally, and culturally. These tendencies toward using sociological characteristic to make determinations about spouse choice has many effects on the lives and livelihoods of those who choose to marry one another, as well as their children and future generations. Within a generation, The tendency of the wealthy and well-educated to marry other wealthy and well educated people helps lead to larger concentrations of wealth within wealthy families as they pool their fortunes through marriage, and the tendency of the poor and poorly educated to intermarry reduces their opportunities for gaining wealth, as their marriages are not the boons to them that the marriages of the wealthy are. This concentration of wealth in families also perpetuates across generations as parents pass their wealth on to their children, with each successive generation inheriting the resources of both of its parents. The combined resources of the parents allow them to give their child a better life growing up, and the combined inheritances from both parents place them at an even greater advantage than they would be with their superior education and childhoods. This has an enormous impact on the development of the social economic structure of a society.