Mate choice in humans
In humans, males and females differ in their strategies to acquire mates and focus on certain qualities. There are two main categories of strategies that both sexes utilize: short-term and long-term. Human mate choice, an aspect of sexual selection in humans, depends on a variety of factors, such as ecology, demography, access to resources, rank/social standing, genes, and parasite stress. While there are a few common mating systems seen among humans, the amount of variation in mating strategies is relatively large. This is due to how humans evolved in diverse niches that were  geographically and ecologically expansive. This diversity, as well as cultural practices and human consciousness, have all led to a large amount of variation in mating systems. Below are some of the overarching trends of mate choice. Female mate choice. Although human males and females are both selective in deciding with whom to mate, females exhibit more mate choice selectivity than males, as is seen in nature. However, relative to most other animals, female and male mating strategies are found to be more similar to each other's. According to Bateman's principle of Lifespan Reproductive Success (LRS), human females display  the least variance of the two sexes in their LRS due to their high obligatory parental investment, that is a nine-month gestational period, as well as lactation following birth in order to feed offspring so that their brain can grow to the required size. Human female sexual selection can be examined by looking at ways in which males and females are sexually dimorphic, especially in traits that serve little other evolutionary purpose. For example, male traits such as the presence of beards, overall lower voice pitch, and average greater height are thought to be sexually selected traits as they  confer benefits to either the women selecting for them, or to their offspring. Experimentally, women have reported a preference for men with beards and lower voices. Female mate choice hinges on many different coinciding male traits, and the trade-off between many of these traits must be assessed. The ultimate traits most salient to female human mate choice, however, are parental investment, resource provision and the provision of good genes to offspring. Many phenotypic traits are thought to be selected for as they act as an indication of one of these three major traits. The relative importance of these traits  when considering mate selection differ depending on the type of mating arrangement females engage in. Human women typically employ long-term mating strategies when choosing a mate, however they also engage in short-term mating arrangements, so their mate choice preferences change depending on the function of the type of arrangement. Short-term mating strategies. Women do not always seek and engage in long-term mating arrangements. This is evidenced by many men's tendencies to seek multiple sexual partners—a trait that could not have evolved if women were not also engaging in short-term arrangements—and by the  tendency of some women to pursue affairs outside of their long-term couple pairings. David Buss outlines several hypotheses as to the function of women's short-term mate choices: Long-term mating strategies. While there has been evidence and research to support the existence of short-term mating in women, it has nevertheless been shown that women prefer long-term partners over short-term mates. This preference is due to women's tendency to invest and require more energy for parental care. In long-term mating arrangements, women typically look for males who will provide a high level of  parental investment, and who can provide resources to the woman or to her offspring. The provision of economic resources, or the potential to acquire many economic resources, is the most obvious cue towards the ability of a man to provide resources, and women in the United States have been shown experimentally to rate the importance of their partner's financial status more highly than men. However, many other traits exist that may act as cues towards a man's ability to provide resources that have been sexually selected for in women's evolutionary history. These include older age—older males  have had more time to accrue resources—industriousness, dependability and stability—if a woman's long-term partner is not emotionally stable or is not dependable then their provision of resources to her and her offspring are likely to be inconsistent. Additionally, the costs associated with an emotionally unstable partner such as jealousy and manipulation may outweigh the benefits associated with the resources they are able to provide. Women's mate choice is not as straightforward as selecting a mate that displays all of her desired qualities. Often, potential mates will possess some qualities that are desirable and some that  are not, so women must assess the relative costs and benefits of their potential partners' traits and 'trade off'. Women's mate choices will also be constrained by the context in which they are making them, resulting in conditional mate choices. Some of the conditions that may influence female mate choice include the woman's own perceived attractiveness, the woman's personal resources, mate copying and parasite stress. Romantic love is the mechanism through which long-term mate choice occurs in human females. Male mate choice. Generally, it is unusual for males within a species to be the choosy sex . There are many reasons for this. In humans, following sexual reproduction, the female is obliged to endure a nine-month pregnancy and childbirth. This means that females naturally provide a greater parental investment to offspring, than males. Human males have a larger quantity of gametes than females, which are replenished at a rate of approximately 12 million per hour. Conversely, female humans are born with a fixed amount of egg cells which are not restocked over the lifespan. This provides males with a greater window of opportunity to mate and reproduce than females, hence females are usually more choosy. Despite  not being the typically choosy gender, human males can be influenced by certain traits of females when making decisions about a potential mate: Short-term mating strategies. When finding a short-term mate, males highly value women with sexual experience and physical attractiveness. Men seeking short-term sexual relationships are likely to avoid women who are interested in commitment or require investment. Examples of short-term mating strategies in males: Long-term mating strategies. Although from an evolutionary perspective women are usually the choosy sex, if a human male has the desire to reproduce he may seek certain qualities in  a potential mate who could be the mother of his offspring. Humans have the ability to rely on biological signals of reproductive success and non-biological signals, such as the female's willingness to marry. Unlike many animals, humans are not able to consciously display physical changes to their body when they are ready to mate, so they have to rely on other forms of communication before engaging in a consensual relationship. Romantic love is the mechanism through which long-term mate choice occurs in human males. Males may look for: Parasite stress on mate choice. The parasite-stress theory , otherwise known as pathogen stress, states that parasites or diseases put stress on the life development of an organism, leading to a change in the appearance of their sexually attractive traits. The initial research on the Hamilton–Zuk hypothesis (see indicator traits) showed that, within one species (brightly colored birds), there was greater sexual selection for males that had brighter plumage (feathers). In addition, Hamilton and Zuk showed that, comparing across multiple species, there is greater selection for physical attributes in species under greater parasitic stress. This has influenced research regarding human mate choice. In societies with a high prevalence  of parasites or pathogens, members would derive greater evolutionary advantage from selecting for physical attractiveness/good looks in mate choice compared to that derived by members of societies with lower prevalence. Humans could use physical attractiveness to determine resistance to parasites and diseases, which are believed to lower their sufferers’ ability to portray attractive traits from then on and limit the number of high-quality pathogen-resistant mates. In cultures where parasitic infection is especially high, members could use cues available to them to determine the physical health status of the potential mate. Regardless of the wealth or ideology, the  females in areas that are more at risk or have higher rates of parasites and diseases would weigh masculinity more highly when rating potential mates. Criticisms. Gangested and Buss (2009) say that research indicates that parasite stress may have only influenced mate choice through females searching for "good genes" which show parasite resistance, in areas which have high prevalence of parasites. John Cartwright also points out that females may be simply avoiding the transmission of parasites to themselves rather than it being them choosing males with good genes and that females look for more than just parasite-resistant genes. MHC -correlated mate choice. Major histocompatibility complex (MHC) or, in humans, human leukocyte antigen (HLA) produces proteins that are essential for immune system functioning. The genes of the MHC complex have extremely high variability, assumed to be a result of frequency-dependent parasite-driven selection and mate choice. This is believed to be so it promotes heterozygosity improving the chances of survival for the offspring. Odor preferences. In experiments using rats, MHC-associated mate choice indicated that odor cues played a role. In humans, there is conflicting evidence about whether men and women will rate the opposite genders odor as more  pleasant, if the potential mate has MHC-dissimilar antigens to them. However, women on contraceptive pills rate the odor of MHC-similar men as being more pleasant, it is unknown why women on contraceptive pills rate smell in this way. It was found that when processing MHC-similar smells were processed faster. Contrary to these findings, other studies have found that there is no correlation between attraction and odor by testing males' odor preferences on women's odors. The study concludes that there is no correlation in attraction between men and women of dissimilar HLA proteins. Research completed on a  Southern Brazilian student population resulted in similar findings that found significant differences in the attraction ratings of giving to male sweat and MHC-difference. Facial preferences. Human facial preferences have been shown to correlate with both MHC-similarity and MHC-heterozygosity. Research into MHC-similarity with regards to facial attractiveness is limited but research so far suggests that women, when thinking of long-term relationships, will choose males who are MHC-similar. While facial asymmetry hasn't been correlated with MHC-heterozygosity, the perceived healthiness of skin appears to be. It appears to be that only MHC-heterozygosity and no  other genetic markers are correlated with facial attractiveness in males and it has been shown that so far that there is no correlation that has been found in females. Slightly different from facial attractiveness, facial masculinity is not shown to correlate with MHC heterogeneity (a common measure of immunocompetence). Criticisms. A review article published in June 2018 concluded that there is no correlation between HLA and mate choice. In addition to assessing previous studies on HLA-Mate choice analysis to identify errors in their research methods (such as small population sizes), the study collects a larger set of data and  re-runs the analysis of the previous studies. By using the larger data set to conduct analysis on 30 couples of European descent, they generate findings contrary to previous studies that identified significant divergence in the mate choice with accordance to HLA genotyping. Additional studies have been conducted simultaneously on African and European populations that only show correlation of MHC divergence in European but not African populations.