Genetics, Human interaction

Sexual attraction – Part 1

Apologies for the late posting of this fortnight’s articles, I needed to wait for an embargo to lift prior to publishing.

Finding a mate is one of the most important tasks for every organism which reproduces sexually. While people say they have different ‘tastes’ in an ideal mate, in fact there is a large amount of commonality between what humans define as attractive, and at the risk of taking the ‘fun’ out of finding a mate, there are specific physiological reasons for sexual attraction.


Attractiveness may have hormonal reasons

One of the major hormones in the reproductive system of women is oestrogen, with the male equivalent testosterone. As well as having vital roles in regulating the reproductive system, they also affect the attractiveness of a person.


A female with high oestrogen levels will usually have a rounded face and large eyes, two factors which are generally regarded to increase attractiveness. When an audience is shown 2 computer generated faces with one having these features but otherwise identical to the first, nearly 70% of people find the “high-oestrogen” face more attractive, with the proportion of women and men who find it attractive being around equal.


Humans aren’t the only animals to have facial features altered depending on fertility state. The face of female rhesus macaques (a type of monkey) darkens when they are fertile, with the changes similarly linked to hormonal cycles. Interestingly, males can actually recognise the fertility states of female partners from these features. The more familiar a male is with a particular female increased their knowledge of fertility states from these cues.


Similarly, a male with high-testosterone will usually have the attractive square jaw and angular face, and when an audience is shown computer generated faces and asked to choose which is more attractive, the “high-testosterone” face comes out as the preferred face, again by nearly 70% of the people. Interestingly however, while the same number of men and women found the “high-oestrogen” face attractive, men were less likely to find the “high-testosterone” face more attractive than the “low-testosterone.”


There is an evolutionary reason for an increased attractiveness as a result of high levels of sex hormones. Women with high oestrogen have high fertility levels, and so are desirable mates due to an increased ability to produce offspring. Similarly, testosterone levels in men are a sign of high quality genes, which are sought after to increase the chances of offspring survival (remember all these processes evolved long ago when living conditions were much harsher). Testosterone in men actually acts as an immunosuppressant – it reduces the activity of the immune system. While this may seem a disadvantage, the ability of the male to have survived to procreation age shows that they must be genetically strong to have been able to overcome any infections with the reduced immune system. For these reasons, high sex hormone levels are considered to be desirable traits in a mate.


Supporting this idea that high testosterone is a sign of genetic strength, when women ovulate they tend to become even more attracted to the high-testosterone males, showing there is an evolutionary mechanism to try to choose mates with ideal genetic characteristics.


Although testosterone is a sign of genetic strength, it does however also increase some undesirable traits when choosing a mate. High testosterone does increase aggressiveness, making the male less nurturing towards offspring, and also increases the male’s desire for what is called sexual novelty; in effect the male will be more likely to be unfaithful as they tend to seek different sexual partners. So for women there is a trade-off to be made between high and low testosterone, to balance the advantages and disadvantages, and it may also explain why some women in particular seem to like “bad boys”, they would like have high testosterone levels. For men however, there is no trade-off, they will generally just seek high-oestrogen (and hence fertile) females.


For these reasons, women may tend towards a lower testosterone male for the reasons of producing offspring as although the offspring may be of lower genetic quality, they may be more likely to be provided for by the father. However, often testosterone levels in men will drop after producing offspring – a mechanism to reduce aggressiveness, promote nurturing behaviour, and increase faithfulness to a single partner. Women’s preferences also tend to change slightly with age, with older females more likely to prefer lower testosterone males.


As these effects have been developed due to evolutionary pressures and are linked to genetics and hormones, they are consistent across cultures. No matter where people come from, they tend to look for the same features in a mate.


Thanks go to Bill von Hippel from the University of Queensland and Rob Brooks from the University of New South Wales

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