Female short-term

David Buss outlines several hypotheses as to the function of women's short-term mate choices:

• Resource hypothesis: Women may engage in short-term mating in order to gain resources that they may not be able to gain from a long-term partner, or that a long-term partner may not be able to provide consistently. These resources may be food, protection for the woman and her children from aggressive men who may capture or sexually coerce them, or status, by providing the woman with a higher social standing. Women may also benefit from having several short-term mating arrangements through paternity confusion—if the paternity of her offspring is not certain, she may be able to accrue resources from several men as a result of this uncertainty.^[5]
• Genetic benefit hypothesis: Women may choose to engage in short-term mating arrangements in order to aid conception if her long-term partner is infertile, to gain superior genes to those of her long-term partner, or to acquire different genes to those of her partner and increase the genetic diversity of her offspring. This relates to what is known as the sexy son hypothesis; if a woman acquires genes from a high quality male, her offspring will likely have higher mate value, resulting in their increased reproductive success.^[5]
• Mate expulsion and mate switching: Women may engage in a short-term mating arrangement in order to cause an end to a long-term relationship; in other words, to facilitate a break-up. Women may also use short-term mating if their current partner has depreciated in value, and they wish to 'trade up' and find a partner that they believe has higher value.^[5]
• Short-term for long-term goals: Women may use short-term sexual relationships in order to assess a mate's value as a long-term partner, or in the hopes that the short-term arrangement will result in one that is long-term.^[5]

Female long-term

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.^[5] 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.^[5]

Women's mate choices will also be constrained by the context in which they are making them, resulting in conditional mate choices.^[1] 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.^[5] Romantic love is the mechanism through which long-term mate choice occurs in human females.^[6]

Male short-term

When finding a short-term mate, males highly value women with sexual experience and physical attractiveness.^[7] Men seeking short-term sexual relationships are likely to avoid women who are interested in commitment or require investment. In short-term sexual relationships, men are less choosy because of low parental investment.

Examples of short-term mating strategies in males:

• Multiple sexual partners: When looking for short-term sexual relationships, men may wish for there to be as little time as possible between each partner.^[7]
• Physical attractiveness: Men who are interested in a short-term sexual relationship are more likely to prioritise information about the body of potential partners, rather than their faces.^[7] When finding a female for a short-term relationship, compared with a long-term relationship, males are less likely to prioritise factors such as commitment.
• Relaxation of standards: It has been reported that men are more likely to engage in a sexual relationship with women who have lower levels of intelligence, independence, honesty, generosity, athleticism, responsibility and cooperativeness, when this relationship is short-term.^[7] Men may be more accepting of lower standards, than what they usually prefer, because they are not entering a long-term relationship with this person.
• Sexual experience: Many men assume that women who have engaged in sexual experiences beforehand are likely to have a higher sex drive than women who haven't.^[7] These women may also be more accessible and require less courtship.

Male long-term

Humans have the ability to rely on biological signals of reproductive success and non-biological signals, such as the female's willingness to marry.^[8] 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.^[6] For long-term sexual relationships, men are usually equally choosy because they have a similar parental investment like the women, as they heavily invest in the offspring in form of resource provisioning.

Males may look for:

• Commitment and marriage: A human male may be interested in mating with a female who seeks marriage.^[8] This is because he has exclusive sexual access to the female, so any offspring produced in the relationship will be genetically related to him (unless the female has sexual intercourse with another male outside of the marriage). This increases the likelihood of paternity certainty. With two married parents investing in the offspring, their chance of survival may increase; therefore the male's DNA will be passed on to the children of his offspring. Also, a male who is interested in committing to a female may be more attractive to potential mates. A male who can promise resources and future parental investment is likely to be more appealing to women than a male who is unwilling to commit to her.
• Facial symmetry: Symmetrical faces have been judged to signal good general health and the ability for a woman to withstand adverse environmental factors, such as illness.^[8]
• Femininity: A feminine face can be a signal of youth, which in turn signals strong reproductive value.^[8] As a woman gets older, her facial features become less feminine due to ageing. Femininity can also be linked to disease-resistance and high estrogen levels, which are factors that suggest reproductive value to a potential mate.
• Physical beauty: Observable characteristics of a woman can indicate good health and the ability to reproduce, qualities which are likely to be desired by a male. This may include smooth skin, absence of lesions, muscle tone, long hair and high energy levels.^[8] Women with darker features (lips, eyes, eyebrows) relative to their facial skin have been found to be more attractive, as this increases facial contrast (the same features appear to decrease male attractiveness).^[9][10]
• Waist-to-hip ratio: A waist-to-hip ratio of 0.7 is an indicator of fertility, lower long-term health risks and suggests that the woman isn't already pregnant.^[8] A male is likely to desire these qualities in a mate, as it will increase the chance of survival of any offspring the couple have together.
• Breasts: The pigmentation of nipples and breasts appears to be the most important quality of breast attractiveness. Men rated women with dark nipples and dark areola as significantly more attractive than those with light-colored nipppes or areola.^[11] Breasts of medium cup size were found to be the most attractive, however authors noted that men focused primarily on the coloration of nipples and areola rather than breast size.^[11]
• Youth: Both young and old men are attracted to women in their twenties.^[12][13] Faces that appear younger are usually rated as more attractive by males.^[8]

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.^[25] 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.^[16]

Odor preferences

In humans, there is evidence that women will rate men's odor as more pleasant if the odor has MHC-dissimilar antigens, which is proposed as a way of avoiding inbreeding and increasing heterozygosity.^[26][27] 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.^[28] 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.^[29] 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.^[30]

Facial preferences

Human facial preferences have been shown to correlate with both MHC-similarity and MHC-heterozygosity.^[31] Research into MHC-similarity with regards to facial attractiveness is limited. One study found that women may prefer mates with MHC-similar faces, despite evidence that they prefer men with dissimilar body odors.^[26] While facial asymmetry hasn't been correlated with MHC-heterozygosity, the perceived healthiness of skin appears to be.^[32] It appears to be that only MHC-heterozygosity and no other genetic markers are correlated with facial attractiveness in males^[33] and it has been shown that so far that there is no correlation that has been found in females.^[34][35] Slightly different from facial attractiveness, facial masculinity is not shown to correlate with MHC heterogeneity (a common measure of immunocompetence).^[36]

Criticisms

A review article published in June 2018 concluded that there is no correlation between HLA and mate choice.^[37] 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.^[38]