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Evolutionary explanations for use of prostitutes by men
We might begin to understand the male propensity to pay for sex, with Parental Investment theory as proposed by Trivers (1972). Trivers suggested that many of the sex differences in human behaviour might be explained by differences in minimal parental investment, including the male propensity to use prostitutes. Females have much greater minimal investment in offspring than men; nine months of pregnancy in contrast to a short period of copulation, and a few teaspoons of semen. The greater cost to women over men - in terms of resources and time - of having children leads us to one prediction, men rather than women should display greater interest in short-term mating strategies (Buss & Schmitt, 1993 ; Clark & Hatfield, 1989 ; Marlowe, 2000). In line with this, a number of studies have shown that, almost exclusively, men are the purchasers of sex. Atchison, Fraser & Lowman (1998) found only two women who were clients of prostitutes. Each of these had participated in a group-sex act with a boyfriend and a prostitute. We can therefore disregard these (rare) instances of women buying sex, since it was reported that the boyfriend initiated the exchange rather than the female.

Furthermore, Shutt et al. (2011) demonstrated that biological sex remained the single greatest predictor of prostitute use even after a host of demographic and criminological variables were controlled for. These variables included: parental relationship quality, delinquency, drug use history, and sexual orientation. A related line of evidence comes from the large differences in the number of sexual partners reported by men and women. Men tend to report the greater number of sexual partners. Brewer et al., (2000) note that once male use of prostitutes is taken into consideration, the observed difference in the number of partners disappears. It has been demonstrated (Sohn, 2016) that men pay younger prostitutes more money than older women. This fits with other research suggesting men maximally prefer women who fall approximately within the range of late teenage years and the late twenties (Anfolk et al., 2015 ; Kenrick & Keefe, 1992 ; Kenrick et al., 1996 . The reason for this is that women in this age bracket have higher fertility than those outside of it. Sohn’s findings suggest that one of the reasons men may pay for sex with prostitutes is to access women of greatest evolutionary value; those possessing youth and related cues to reproductive value.

An analysis of online escort advertisements (Griffith et al., 2016) lends support to the view that men will pay the highest price for sexual access to a highly desirable female. Aside from youthfulness, other short-term male sexual preferences are for a waist-to-hip ratio of around 0.7, a low BMI and large breasts. It was found that all three of these preferences led to higher fees for women who possessed them. This fits with earlier research suggesting an economic or market-based dimension to the price of sexual access (Edlund & Korn, 2002 ; Baumeister & Vohs, 2004 . Here, women possessing highly desirable characteristics can attach the highest price to sexual access. Both Sohn (2016) and Griffith et al., (2016) suggest that their work allows us to draw one further conclusion about the reasons men buy sex from prostitutes. They may be otherwise unable to attract highly desirable mates. A number of interview studies (McKeganey & Barnard, 1996 ; Monto, 2000 ; Peng, 2007 support this assertion. There is also convergent experimental evidence (Buss & Shackelford, 2008) that the women men find most desirable are the most selective when choosing mates. This notion is somewhat controversial, and requires further experimental verification. There is little useful consideration of male homosexual prostitution from an evolutionary perspective.

Prostitution and Social Exchange Theory
Prostitution can be identified as a form of social exchange, a female resource where the female typically gives the male sex in exchange for money (Baumeister & Vohs, 2004 . A general consensus is that males receives more in return for their money, as presumably, prostitutes are less likely to refuse sex in comparison to non prostitutes. However, it may be that the sex is of a lesser value as prostitutes are not seen as high quality sex partners. In terms of social exchange theory, as a result of having had sex with many people, the prostitute’s value of their sexual favours is lowered.

There is support for the view that the price that one pays for sex is associated with the females’ general economic situation. In countries where poverty is greater, prostitution is also found to be higher such as Cuba and Southern Asia; women from these countries may be forced to engage in prostitution as a source of income, they may feel obligated to offer sex at a lower price than usual in order to get the desired resources. Yet at the same time, it is still essential for females to maintain a high price of sex because if they are attractive and display healthy characteristics such as good skin and a low waist to hip ratio, they are more likely to be preferred by males seeking to have sex

Within prostitution, women comprise the supply and men constitute the demand. Thus, in countries where there is an excess of prostitution, men may able to obtain sex at a much lower price as the supply exceeds the demand. From an evolutionary perspective, the aim of sex is for the female to become pregnant and produce healthy offspring; with fewer men around, it would be expected that pregnancy rates would be lower too. However Barber (2000) found the opposite in a cross-cultural survey of 185 countries, in that teenage pregnancy was high when men were scarce. A reason for this may be because when there is a shortage of men, females compete against each other and offer sex in exchange for fewer resources so in terms of prostitution; they would have sex for a lower price. On the other hand, the price of sex is more likely to increase when the demand for prostitution is higher but there is limited supply of it.

Prostitution displays in the animal kingdom
Prostitution is considered an anthropomorphic term and observations of such behaviours within non-human animals may be displayed differently with alternate functions and intents (Zuckerman,1999). It has led to the rise of varying definitions with some saying animal prostitution involves liberation of sexual responses from reproduction, thus one should focus on the results of engaging in behaviours rather than the process (Zuckerman,1999 . Others define the term more specifically such as in the context of primates where it’s assumed for them the process is more sophisticated. It’s stated that in their case, it’s about essentially giving sexual favours to get physical protection and economic advantages (Kempf, 1913) . There are not many studies looking at animal prostitution (chimps, monkeys and penguins are the most studied), with first observations being relatively recent, and so it’s not clear if females engaging in courtship behaviour with multiple males is innate or down to social learning.

Primates

One explanation for animal prostitution is the meat – for sex hypothesis which has gained mixed support. This states that women avoid the costs associated with hunting (such as foraging and searching) and will provide sexual favours to an unrelated male to receive food from the hunt (Boesch and Boesch-Acherman, 2000 . This benefits the male as he can increase the likelihood that he’ll mate with her and extend his progeny. Female chimpanzees can gain adequate nutritional intake without putting herself at risk through mating with more males to receive more food. Usually Chimpanzees and monkeys are observed to see if this holds true due to their genetic closeness to humans and the fact that chimpanzees are a promiscuous group with lots of female mate choice (Hasegawa and Hiraiwa-Hasegawa, 1983. Male chimps will attempt to monopolize on this, suggesting that female choice plays a crucial role in the male’s reproductive success and that females have the option to mate with multiple males. There are similarities and differences often seen between different chimpanzee communities. In the case of tai chimpanzees, it’s been observed that males will only gift food to females they want specifically to and did mate with, implying that meat is offered literally for sex only (Gomes and Boesch, 2009). On the other hand, Ngoge and Gombe esterous chimpanzees did not receive special treatment compared to anestrous ones in terms of frequency of mating and the amount of food given. Reproduction may then not be the main reason for meat being exchanged as male chimpanzees did not focus their resources on female with whom they could mate with. More robust and long term findings were seen in a study of 20 tai chimpanzees in Côte d’Ivoire's Taï National Park. Here, female chimps were more likely to engage with male chimpanzees if they offered food at least once, indicating that sharing meat with females improved a males’ mating success (Boesch and Boesch-Acherman, 2000 . The food for sex barter occurs over a long period due to males building trust with females over time through providing them with meat. When variables such as male rank, age and rank of female were controlled to see if friendship was not the reason for this, it was seen the relationship between copulation frequency and meat sharing was still significant. This contradicts previous research on human male mate preferences who are more likely to choose young women to mate with . The specific mechanism involved that leads to transferring meat from the male to female is not fully known. But, such results do show that due to the parallels in primate and human societies, a male showing his ability to provide nutrition for the female and future offspring can increase chances of mating and reproduction. In turn, women are giving greater mate choice and can engage in copulation with many males to gain the food she requires (Stumpf and Boesch, 2006 . An alternative view to the ‘meat for sex’ hypothesis is the ‘biological market theory’. The amount a male is willing to pay depends on the scarcity of the commodity (in this case females) present. Male Macaque monkeys pay for sexual access to the females, through trading grooming services, as a result female sexual activity doubled (Gumert, 2007. If there are more females available, the less effort males would put into providing grooming, therefore the amount he invests depends on how many options he has. Therefore, being one of few female monkeys in a groups means she is more likely to engage in promiscuous behaviour as she likely to receive more and better quality grooming from the competing males.

Penguins

It’s not unusual to see exchange behaviour in many bird species in the form of exchanging sex for food and resources (Brown, 1967 . Female birds are more likely to copulate if they receive food from a male they’ve had sex with, however for many species this is kept to monogamous courtships (Tasker and Mills, 1981) . One of the first studies that came about looking at animal prostitution was conducted by Hunter and Davies (1998) during the Adélie penguin’s prelaying period which lasts from mid-October to November (Spurr, 1975b) . They live in large colonies in the Antarctic, where monogamous and extra-pair copulations are evident. These penguins usually stick to the same mate and will build nests together. Problems come about when there are not enough stones available to build and maintain nests essential for survival, due to them being in demand (Sladen,1958) . Stone collection is said to improve nest quality and increase reproductive success (Moreno, Bustamante and Viñuela, 1995). As a result, females will engage in copulations with males they find in order to acquire resources that will help them build nests. This will sometimes occur even if she has a mate, thus ‘sneaking’ around. The process is said to be mutually beneficial for the female and ‘single’ male. She gets the stones she needs and he is able to have offspring that are taken of by another male bird, meaning he will not have to provide parental investment. This process is often deemed safe enough as she enters his nest and engages in mutual courtship behaviour, as a result the male is not aggressive towards her and so she is not under any threat. Initially, the male birds will be sexually frustrated due to being unable to find a mate. They collect the best stones they can find, utilising them as currency and use them barter for sex. They are essentially ‘tricked’ by the female as they are under the assumption that copulation means that a long term relationship will begin. However, once stones are obtained by the female after showing courtship behaviour, they will leave and return to her pair-male and their nest. Other tactics used can involve the female engaging in a courtship ritual, but then she leaves before copulation occurs However, the females often only take 1 or 2 stones, when 100’s are needed to build a nest. Because of this, it’s suggested that the females engage in courtship for pleasure too alongside getting their needed stones. This may happen because she is first building his trust and will then return to the unsuspecting male’s nest to collect more stones under the guise of wanting to engage in sex again. Others suggest she does this to show her pair – male the reason for her absence and so prevents him from engaging in retaliatory copulation (Birkhead and Moller, 1992) Observations in animal species can provide a better understanding if such theories of prostitution hold true and if their explanations are relevant today. Plus, evidence in animal species can potentially be applied to similar behaviours in humans, suggesting that prostitution could be a universal, thus evolved mechanism.