Mhc and Sexual Selection

SHE'S JUST NOT THAT INTO YOU: Problems With MHC and The Pill

ABSTRACT

Sexual selection theory states that males maximize reproductive success through extra pair copulations while women do this by discrimination of male genetic quality or simply "choosiness". The Major histocompatibility complex which is involved in immune system function can be detected through olfactory senses and also signals genetic quality of a person. It has been proven that those with MHC heterozygous alleles have a higher genetic quality because their immune system function is higher and therefore gives rise to phenotypic effects such as skin healthiness as seen in Roberts 2005. It is also noteworthy to conclude that because MHC heterozygosity is optimal, many women prefer MHC heterozygous men and offspring and therefore generally are more attracted to those with MHC alleles that are most dissimilar to them. However in recent studies this trend is seen to be reversed by the birth control effect which causes females to mimic pregnant women's MHC preferences and have a higher attraction to MHC similar mates. Due to the increased use of the contraceptive pill, this poses a problem to women on the pill regarding mate selection and their relationships.

INTRODUCTION

Whether we realize or not, smell plays an important role in human mate selection. Sexual selection theory states that males maximize their reproductive success through extra pair copulations while females use discrimination of mate quality as a means to maximize their reproductive success. In light of this, certain studies suggest that smell indicates biological quality and therefore it is no surprise that females would use this as a discrimination factor. Nevertheless, Capparuccini concluded that "the olfactory system can have a dominant effect on the visual system when related to sexual behavior." (pg 1327), likewise, leading evolutionary psychologist Craig Roberts claimed that "odor perception plays a significant role in maintaining attraction to partners." (Henderson) This suggests that smell can ultimately influence a person's attraction to a partner.

Found in most vertebrates (like humans) is a large genomic area that plays an important part in immunity called the major histocompatibility complex (MHC). In Humans this is also called the human leukocyte antigen system (HLA) which is located on chromosome six. The MHC is responsible for protein synthesis and provides a wide array of peptides; this is mainly because the MHC locus is polygenic. These MHC proteins act as an alarm system to alert the cell of any foreign substance. MHC genes are highly polymorphic and numerous alleles have been discovered. Several MHC genes are codominantly expressed which explains why allelic diversity is so great in the MHC. The major histocompatibility complex molecules can be detected through pheromones, aiding the concept that the MHC plays a part in the selection of a mate through olfactory senses.

In several studies, it has been found that humans, like other vertebrates, can detect a MHC-specific odor. In 1983, Hedrick and Thomson discovered that in human populations, heterozygosity of the MHC is more frequent than expected by chance, and Wedekind and Furi speculate this result seems to occur from natural selection as well as sexual selection, implying that to some extent, humans use MHC odor as a factor in mate selection. (Wedekind & Furi, 1997, pg 1471) In their 1997 study, Claus Wedekind and Sandra Furi performed an experiment to test the hypothesis that heterozygous MHC-allele combinations are more advantageous under selective pressure by pathogens. They aimed to discover if the mate preference was based on creating MHC-heterozygous offspring or specific MHC allele combinations that were beneficial for the environmental conditions of the time. They discovered that the general trend was that "pleasantness" of odor was negatively correlated with the degree of MHC similarity between the t-shirt smellers and the t-shirt wearers in both sexes and were not aimed at specific MHC combinations.

However the most important discovery of Wedekind and Furi's experiment was that women who were taking a contraceptive pill did not display the same results as the men and women who were not taking a contraceptive pill. In fact, women on the pill displayed the opposite trend in which "pleasantness" of odor was positively correlated with the degree of similarity between the woman and the t-shirt wearer. This poses a great problem in mate selection for women, especially with the significant increase in birth control use since its legalization and mass production. Essentially, birth control is acting as a disruptor of a mate selection process and producing the opposite of the desired results. However, the question to be explored here is what sort of affect does this have on mate selection and attraction, personal relationships, and populations as a whole when a women stops taking the birth control pill.

IMPORTANCE OF MHC

The major histocompatibility complex is involved in immunity and autoimmunity because MHC proteins act as signals to alert the cell that there is a foreign or "invader" substance inside the cell. However, the MHC is also involved in mate selection because MHC genes give off a certain odor that can be detected by others. Based on these odors the genetic similarity or dissimilarity between two people can also be detected subconsciously according to its "pleasantness" to the other person. MHC genes prove to be good indicators of a person's genetic quality because of the phenotypic effects and almost always, those who are heterozygous for MHC alleles show a greater physically healthy appearance. This is because those who are heterozygous for MHC alleles are less susceptible to disease and have a higher immune system function. This could serve as a good reason for people's natural MHC allele preference.

In Craig Roberts's 2005 study, "MHC-heterozygosity and human facial attractiveness" the interesting correlation between MHC allele's and facial attractiveness is explored. In the first experiment, ninety-two men between the ages of 18 and 31 were genotyped at the three most polymorphic MHC loci, sixty-nine men were heterozygous at all three loci while twenty-three were homozygous at one or more loci. Digital photographs of the men's faces were taken with no cues to hair style, facial hair, or other disruptive factors and presented to 50 women aged 18 to 49 who then judged these photographs on attractiveness. Photographs of skin patches from the men's right cheek were taken and presented to the women who then judged these photographs