Have you ever wondered why some individuals in a relationship resemble each other?
Biologically speaking, people have a tendency to be more attracted to someone with similar phenotypes. When someone ends up with a person who looks like they could be long-lost-siblings, it is called phenotypic matching (or arguably, just plain narcissism.) You probably are just dying to know how then does a person insure immunogenetic diversity for their offspring. (I think about these things all the time, don’t you?) Well, luckily, the body has a natural system of being able to detect immunogenetic diversity through olfaction, or smell.
From a Darwinian “survival of the fittest” standpoint, it is deleterious to reproduce with someone that has a similar genetic constitution because it increases the chances of producing non-viable offspring as a result of potentially harmful homogeneous repetitions in the genome. (Translation: Don’t have babies with your relatives because they will be messed up.) If people have a tendency to phenotypically match their mates, then how do people avoid having similar genetic makeup as their mates? Furthermore, how do people insure they are not related to their partners, if they look so much like them?
Cultures all over the world vary in their complexities and beliefs, but one thing that almost every culture has in common is the incest taboo. There are several theories about the occurrence of the universality of the incest taboo. The most popular is the Westermarck theory, which essentially states that individuals who grow up in close proximity to each other and interact on a regular basis develop a natural sexual aversion to each other.
One case that supports the Westermarck theory is demonstrated by the high percentage of unsuccessful marriages in the Chinese minor marriage system. Chinese minor marriages are a form of arranged marriages in which a family adopts a “little daughter-in-law” who is raised alongside her future spouse. These marriages end in a remarkably high divorce rate and produce little to no offspring. The Westermarck effect explains this occurrence and the universality for the tendency of natural incest avoidance.
As I mentioned earlier, the body has a natural mechanism that detects immunogenetic diversity. The major histocompatibility complex (MHC), a genomic region (little bunches of genes) found in vertebrates, is nature’s mechanism for inbreeding avoidance. The MHC is largely responsible for autoimmunity and immune system functions (or, it helps to fight off diseases and stuff).
Diversity in the MHC region is essential for the immunity diversity of a population. The more the genes in the MHC vary, the better they can fight the various kinds of microbes to which the body is exposed. Furthermore, reproducing with someone who has dissimilar MHC from you will produce offspring with augmented immunity variance and overall increased viability. (Super babies — well, just really healthy ones.)
It has been hypothesized that MHC diversity is detected via olfaction (through your nose). This olfaction process enables mate-seekers to detect genetic variety. Therefore, mate-seekers would not be attracted to the smell of potential relatives or people with similar genetic makeup because of the similarity in the MHC regions. It would be evolutionarily advantageous to pick a partner that is immune to different things than you are because your kids would have increased immunity. (Kids are gross in general, but snotty-nosed sick kids are uber gross.)
If I may insert personal anecdotal evidence, my sister once dumped a guy because he “smelled like he was a relative.” Whether or not he actually was related didn’t really matter, it may have been her keen ability to detect immuno-diversity. Although, I must clarify, most people cannot actually detect a specific smell — it is usually just a hormone response that triggers attraction or aversion. So ladies (and I say ladies because it has been argued that only women have the olfactory ability when it comes to mate selection), if your man has bad BO, you might want to reconsider things — especially if he smells like your brother.
Or you know, you could just get him some Old Spice to make your man smell like the man your man could smell like… or something.
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