Donald Symons once said “with respect to human sexuality, there is a female human nature and there is a male human nature and these natures are extraordinarily different” (Symons 1979, p.11). Even though the difference between the sexes with regards to sexual behaviour, mate preferences and desires has been purported to be exaggerated (Stewart-Williams and Thomas 2013), one category of behaviour where extreme differences in behaviour appear to exist is in the charging of, and the willingness to pay for, sexual services. The former is almost invariably the domain of women with the latter restricted almost entirely to men (Bonnerup et al. 2000; Burley and Symanski 1981; Symons 1979). Indeed, a 2014 global report reported that an estimated 95% of sex workers are women worldwide (All-Party Parliamentary Group 2014). Such a disparity could be explained due to the fact that men possess a desire for greater numbers of sexual partners and to uncommitted, casual sex (Buss and Schmitt 1993; Clarke and Hatfield 1989; Schmitt 2003). Although arguably more costly to women than men, benefits can logically accrue to women who have over evolutionary time pursued short-term mating strategies (Greiling and Buss 2000). That said, Symons (1979), when focussing on the evolution of the desire for sexual variety, noted that one of the very few reasons why a woman during our evolutionary past would have sexual intercourse with a man other than her spouse was because she may benefit by trading sexual intercourse for meat, supplies or services. Symons argued that the concept of paying for sexual services from prostitutes is connected primarily to men as sex is widely known to be something that women have and something that men want. Clearly, prostitutes and escorts engage in casual uncommitted sex but they are rewarded by men who are willing to pay for such services. Often referred to as the ‘oldest profession’, prostitution has been recorded in all cultures and across all historical epochs (Bullough and Bullough 1996). The reason why women offer sexual services far more frequently than men may also be because women due to higher obligatory parental investment (Trivers 1972) focus more attention on wealth in men. This has been shown to be important for women cross-culturally and across historical time (Buss 1989; Buss 2012; Symons 1979).

Men’s universal preoccupation with youth and physical attractiveness in women (espoused by evolutionary psychologists as reflecting evolved attraction to cues revealing fertility and reproductive value) has been revealed by studies exploring, for example, preferences for body-shape (Puts 2010; Singh 1993), age preferences advertised in personal ads (Kenrick and Keefe 1992) and online dating (Dunn et al. 2010). Dixson et al. (2011) using an eye-tracking procedure found that men paid particular attention to the breasts on a woman’s body more than any other part with mid-riff and waist being fixated on secondly. It is argued that these features are clear indications of age and fertility (Dixson et al. 2011; Johnston 2006; Puts 2010). Why do men prioritise these features when considering a sexual or romantic partner? Fisher (1930) was the first to propose the idea of reproductive value (RV) by calculating the average contribution an individual woman may have in terms of future reproduction at any age. Thornhill and Thornhill (1983) stated that RV would peak in women in their mid-teens and decline thereafter. Moreover, Buss (2012) concluded that a woman’s RV will ultimately be zero by the age of 50. Symons (1979) proposed that regardless of the nature of the relationship (long-term vs. short-term), men will generally find younger women more attractive because they possess higher RV than older women. It is worth pointing out, however, that fertility differs from RV in that it is defined as the probability of the occurrence of pregnancy following a single coital act (Buss 1989; Symons 1979) which is said to peak in the mid-twenties again declining with age (Thornhill and Thornhill 1983). Research conducted by Montagu (as cited by Symons 1979) suggested that for men, when seeking women to be solely short-term sexual partners, the most attractive age range for them in order for sex to translate into reproductive success would be 23–28. Wood (1992) supported this and argued that fecundity, which is similar to fertility, peaks in the mid-twenties and rapidly declines as a woman approaches menopause. Buss (1989), however, found that men favoured a mean age of 24.83 in a woman, when seeking long-term relationships such as marriage. The targeting of youth by men has also been shown in studies exploring the number of responses women receive to ads placed in personal columns. de Sousa Campos et al. (2002) found when exploring Brazilian dating behaviour that younger women received more responses to advertisements than older women whereas the reverse pattern was observed for men. Such findings lead to the prediction that when seeking sex for payment, men will target younger and more attractive women when paying for sexual services. Women’s tacit knowledge of the preeminent status afforded to their physical attractiveness by men should arguably translate into higher mate value in younger compared to older women. Mate value clearly relates to self-esteem. Wade (2000) found that fecundity-related aspects of the body (a sex appeal subscale) predicted women’s self-esteem. In the case of woman escorts, this should manifest in higher charges/rates made by younger compared to older sex workers.

The Internet has become a place where individuals can meet both romantic and sexual partners (Downing 2012). As of 2015, out of the 54 million singles in the USA, 41 million have tried online dating (The Statistics Portal 2016). Research using online dating sites presents researchers with the opportunity to observe mating preferences in current dating behaviours and to test evolutionarily based theories and hypotheses (Dunn et al. 2010). Online dating has indeed become a considerable business with an estimated value in excess of $2.2 billion a year. As mentioned earlier, the Internet does not solely cater to those interested in forming long-term relationships, and this has been the case for some time. Brym and Lenton (2001) found, for example, that 53% of men used online dating sites in order to pursue sexual only relationships, compared to 20% for women. This supports the idea that casual relationships are more tailored to men (Lippa 2007; Symons 1979). However, with the Internet developing quickly, it is no surprise that markets such as pornography and the solicitation of sex for payment have become more accessible (Brickell 2012). Indeed, in relation to the market of illegal goods, prostitution was one of the first of these markets to develop online (Cunningham and Kendall 2011) and has continued to grow as a means for advertising sexual services (Brickell 2012). Prostitution has recently been found accountable for contributing over £6 billion to the Great British economy (Office for National Statistics 2015) suggesting that as with online dating, it can be considered to be substantially involved in the world economy. Though research using online dating to explore mate preferences is extensive (Dunn et al. 2010; Eastwick and Finkel 2008; Hitsch et al. 2010), no substantive, cross-cultural research project exploring prostitution from an evolutionary perspective has thus far been undertaken.

The current study aims to observe and analyse age differences in charges that escorts advertise online for sexual services. Previous research has tended to label sexual service providers as ‘prostitutes’; however, in this study, they will be labelled as ‘escorts’. The current study endeavoured to test the hypothesis that younger women (those possessing higher mate value) will charge more for sexual services than older women in the knowledge that men will be more willing to pay elevated prices. Based on this theoretical position, it is hypothesised that younger women (ages 20 and 25 in the UK, or age groups 20–30 and 20–34 in the non-UK samples) will advertise higher fees for sexual services than their older counterparts (30, 35, 40, 45 and 50 for the UK, 30–39 and 40–50 for Ireland, Australia and Eastern Europe and 35+ for the USA).