Human mate choice

Natural selection is based upon survival of the fittest. Certain members of a species posess biological traits that mean they're more likely to survive or win out in competition over limited resources such as food, shelter, and particularly mates.

Biological fitness is measured in terms of reproductive success, which ensure long term genetic survival. One would not expect species to posess traits that compromise their chance of survival. Yet, many species do. The male peacock has a huge train of feathers that makes it easier for tigers to catch. It was hypothesised that this occured due to (1) peacocks evolved their train of feathers to intimidate other males or (2) peacocks use their feathers to attract females.

Darwin (1872) proposed a complementary process to natural selection. Sexual selection refers to traits that promote mating and thereby reproductive success, sometimes even at a cost to personal long term survival (e.g peacock's feather train).

Darwin (1872) suggested that sexual selection could explain certain types of sexual dimorphism (the fact that males and females of most species differ from one another in terms of certain physical or behavioural traits).

The males of many species differ from females in that they are physically bigger. Darwin suggested that greater size may have been selected for because large males win out in male-to-male competition over mates. Male elephant seals can be 3 times bigger than females. The males enter into violent battles to become a "beach master" and gain exclusive sexual access to a harem of females.

Darwin (1872) also suggested that males may possess certain features because females prefer them and are preferentially mating with the males that possess them. Females tend to be choosy, but males take whatever they can get. Bateman (1948) suggested that in species where males invest little beyond mating, there is often high variability in the males' mating success than in the females'. Females can only become pregnant once, while the males' chances of having offspring increases with each new female they mate with. Hence males, but not females potentially benefit from every extra acts of mating. Males need to fight off competition so they can access females. Females generally don't need to fight one another to mate, because he is able to mate again and again.

Trivers (1972) offered a reason why females are often choosier than males in selecting a mate. He argued that females have more to lose if they copulate with a poor quality male because they make a greater parental investment.

Females produce many fewer sex cells than males - females produce about 400 eggs in their lifetime while males produce millions upon millions of sperm. Hence, as Trivers put it, eggs are expensive, sperm are cheap.

Once a female's eggs have been fertilised, she must gestate them for a period of time and then sometimes raise the resultant offspring without the help of the male. Therefore, choosing an inferior male is potentially costly for the female, but much less so for the male.

Clutton-Brock & Vincent (1992) criticised the concept of parental investment because it is difficult to measure scientifically. It is virtually impossible to objectively measure the relative investment of male red deer fighting for and guarding a harem of females versus gestating and raising fawns.

Sexual selection is driven by a case of supply and demand. Whichever sex's reproductive potential is in relatively short supply, becomes a limited and thereby valuable resource for the other sex. Potential offspring reproduction rates can be worked out mathematically in terms of maximum numbers of mating required for maximum potential offspring produced.

Male seahorses carry the eggs of the female in a special brood puch. Vincet et al (1992) showed in monogamous species, males fought each other for mates. However, in certain polygamous species, one finds a sex role reversal, in which females compete with one another for access to males or with seahorses, precisely their pouch.

In some species, e.g emperor penguings and common marmosets, males and females work together to raise their young. Males that help raise offspring do not benefit from lots of extra mating unless they can "cuckold" other males, i.e commit adultery with the mate of another male and leave the other male to help raise their young. Thus, males suffer from "paternity uncertainty". While a female always knows that her offspring are hers, a male cannot be so certain. Some studies found actual paternity discrepancies rates of up to 30% (Goetz & Shackleford, 2009). Thus, a third of men tested were raising children than were not their own.

The part of Darwin's theory that people initially most disagreed with was female choice. This is the idea that males may possess certain features simply because females find them attractive. Female choice has been characterised in terms of "good genes" or "good taste". Sexually selected traits based on advertising good genes are honest signals of health. Sexually selected traits that are based upon good taste are traits that are preferred by the opposite sex due to sensory bias or runaway sexual selection.

Zahavi (1975) proposed what he called handicap theory. He reasoned that costly features such as long tails act as a handicap. Thereby, they are an honest inidcator of good genes. In a sense, the males are advertising themselves saying, "My genes are so good that I can afford to produce this luxuriant pointless handicap". Evans & Thomas (1992) manipulated the tail streamers of scarlet tufted malachite sunbirds. Males who had a lengthened tail or had weights added to the tail caught less insects. Shortened tails increased feeding efficiently. Therefore, possessing long tails is a handicap in terms of feeding efficiency

Hamilton & Zuk (1982) suggested that extravagant features (like a long tail or feather trails) might not only act as a handicap but also aindicate parasite resistance. They found that tail lengths in swallows correlate with resistance to mites.

Fisher (1930) argued that once a preference gets established in females it could become self-reinforcing. If females with to produce sons that are viewed as attractive by other females they must continue to choose mates that exhibit the preferred trait. This can lead to runaway sexual selection.

Brain size in hominids tripled in one to two million years. Miller (2000) argued that the rapid expansion of the brain was due to female choice for intelligent males. They preferred males who were witty and creative

Human males have:

• greater upper body strength
• more facial and bodily hair
• greater height and mass
• deeper voices
• risker life histories and higher juvenile mortality
• later sexual maturity
• earlier death
• broader and more prominent chins
• lower levels of fat deposited on buttocks and hips

Sexual competition is more intense in polygamous societies (one male mates with several females) which leads to greater dimorphism in body-size. The body-size dimorphism in our hominid ancestors suggests that they were once strongly dimorphic and therefore probably polygamous. Modern body-size sexual dimorphism suggests we were selected to be mildly polygamous or serial monogamists

It has been estimated that Stone Age women could have raised only 3 to 4 children to adulthood. Parental investment in helpless human babies is so high, its argued women would have needed the help of a mate. Therefore, males would also have made a considerable PI and both sexes would have benefited from their fussy mate choice.

Women have evolved to seek mates with good genes, a willingness and ability to supply resources, and help with childcare. Men have evolved to seek fertile mates with high reproductive potential, good genes and likely to be faithful/ Since both sexes invest heavily in childcare, they would be expected to overlap in the criteria they use for choosing a mate, however it should be slightly different due to males higher potential reproductive rate and paternity uncertainty. 

Buss (1989) administered a mate choice questionnaire to 37 different cultures. Men valued physical attributes more than women in 34 of the 37 cultures. Men preferred women younger than themselves in all 37 of the cultures.

Males should be attracted to females that appear fertile and physically capable of caring for children. Females' period of fertility is from 13-45, and males' fertility is 13-65, lasting 20 years longer than females. Therefore, men should be fussier than women about age and physical features that indicate youth and fertility (Buss, 2014).

On average, men prefer to to marry women who are 24.83 years old when they are 27.49 years old. Women prefer to marry men who are 3.42 years older than themselves. Age at marriage data was collected for 27/33 countries sample by Buss (1989) - the actual mean age difference was 2.99 years, men being older.

It is hypothesised that women prefer slightly older men because it is an advertisement of fitness and health to have reached that age. More hypotheses include: age is an indicator of status and males mature sexually later than females. However, females must not choose a male who is too much older because he must still be able to provide resources and support to help raise their young (Cartwright, 2000).

There is better evidence for why men prefer younger women. Evolutionary theory predicts that what men desire in a mate is not youth per se, but features associated with a high reproductive value. Men will value younger mates because women have a narrower window of fertility, and men must maximise their reproductive potential.

Although, this presents a problem; adolescent males should be attracted to older women because slightly older women have already entered the window of greatest fertility, while girls younger than themselves are yet to do so. Kenrick et al (1996) found that adolescent boys do have a great preference for older women, which seems to support the evolutionary theory.

According to evolutionary theory, physical attractiveness (as an indicator of youth and fertility) should be somewhat more important to males than females. When we choose a mate, we assess overall body shape and facial features.

Waist-to-hip ratio (WHR) in healthy women lies between 0.67 and 0.8, which is an hourglass shape. Singh (1995) tested a number of different cultures. Indonesian, Afro-American and Caucasian. He tested men and women. All groups preferred 0.7. However, Yu & Shepherd (1998) tested the Matsigenka indigenous people of South East Peru, who are relatively free from Western influence and the men preferrd the figure with a WHR of 0.9. The figure of 0.7 was judged to be suffering from fever and having lost weight around the waist. There is a lot of controversy in regards to the roles of WHR versus BMI. Swami & Tovee (2005) argued that BMI as an indictor of economic status is more important than WHR.

Fat distribution in humans is sexually dimorphic. Among hominoids, this feature is unique to humans. After puberty, women deposit fat preferntially around the buttocks and thighs, while men deposit fat in the upper body regions, such as the abdomen, shoulders and neck. Low WHR is indicative of high oestrogen levels. A high WHR indicates a high level of testosterone.

It has been suggested that the average of a trait would tend to be optimally adapted, since the mean of a distribution presumably represents the best solution to the adaptive problem. Thornhill & Gangestad (1993) found that protein heterozygosity tends to be highest in individuals with average traits. It could be that facial attractiveness is an indication of increased parasite resistance. Symmetry is also associated with parasite resistance. However, Perrett et al (1994) found that computer generated perfectly symmetrical faces were judged as less attractive than natural faces with a slight asymmetry.

Perrett et al (1998) found that female faces were judged as more attractive if they were feminised. Feminised features correlate with the baby schema: large eyes relative to face size, thinner jaws, and shorter distances between the mouth and chin. These are signs of youth.

The baby schema: babies of many young posess similar infantile features. We find these "cute" and they trigger a caring response - that is the baby schema. Features are a rounded head shape and protruding forehead, chubby cheeks and upturned nose, large eyes, and large head to body ratio.

Feminised male faces are also judge to be more attractive. This could be a mechanism for looking for long-term male partners. High testosterone could indicate social dominance, but could also indicate an unwillingness to invest in a relationship.

Penton-Voak et al (1999) found that women preferred more masculine faces during the fertile peak of their menstrual cycle, which is when they tend to be more interested in short-term sexual liasons.