Last post talked about the effects that the sex hormones oestrogen (females) and testosterone (males) have on attractiveness to the opposite sex. While the do have a major role in assisting our search for appropriate mates, there are other factors.
The effects of sex hormones on facial features has already been described, however it was believed for a long time that the waist-hip ratio was a prime determining factor for measuring attractiveness of women. Body fat is an indicator of fertility – too little or too much reduces fertility, possibly explaining why very thin or overweight people are not considered generally attractive. A ratio between waist and hip size of 70% has been thought of as optimal. However, waist-hip ratio is an overly simplistic way of determining physical attractiveness, and it is now considered to be a combination of 25 measures which describe physical attractiveness, one example being leg length.
This idea of physical measurements determining attractiveness is not confined to humans. The female widow bird for example will preferentially seek a male with a longer tail. In fact if the male birds' tail is artificially shortened, they will then be less attractive to females. So while there are thought to be around 25 measurements that define human attractiveness, the widow bird's tail is the major determinants of their attractiveness.
Gene matching
In nature there are instances where animals will preferentially seek out animals which have similar genes. The Gouldian finch is found with one of a number of head colours, and when it comes to mating will prefer to partner with another animal of the same head colour. In fact, if it mates with an animal of different colour, the bird will get rather stressed out about it. This mechanism exists to maintain a proper balance in the gender of the offspring, mating with a different head-colour bird will produce more male offspring, so this distaste for mating with different coloured birds has evolved to maintain proper population size and gender ratios.
In humans it is somewhat similar; we tend to look for someone who is similar and not too different from ourselves as that means our genes will be a good match. However, we also look for someone who isn't exactly the same, there is a balancing act involved, and there are several mechanisms to help that. For example, part of the immune system is called the major histocompatibility complex, or MHC, which is slightly different from person to person. Women can actually (subconsciously) detect the MHC-type of a male, and will preferentially choose a male who has a slightly different MHC than their own. This is for two reasons, firstly some similarity means the two people are genetically similar, and secondly, a slightly different MHC will mean their immune system is slightly different, giving offspring a potentially greater variety in their immune system. Strangely however, women on the pill are unable to pick up a male's MHC.
These features which are attractive between men and women are also present between homosexual couples. What straight men look for in women, gay men look for in men. Similarly, gay men actually prefer the smell of gay men over the smell of a straight man, so there does appear to be a difference in pheromones.
These olfactory mechanisms play an important part in maintaining genetic variability by acting as an incest avoidance system. There are several mechanisms which exist, not only in humans, to maintain genetic variability by effectively finding someone closely related 'unattractive', including the MHC-type detection. Again, humans look for someone who is similar but not the same, and different but not wildly different, so these mechanisms exist to maintain that divergence.
Love and sexual attraction can be hard to define and quantify, and a large reason comes down to the physiological mechanisms which have evolved over many generations and have become ingrained. Cues such as hormonal cycles which subtly change the appearance and attractiveness of someone, subconsciously assessing someone's fertility, or detecting the MHC-type of a partner play a role which we never notice and never consider. But nevertheless they're there and while other factors such as personality play a role, inevitably it's these unconscious reasons which will determine whether or not we find someone attractive and an ideal mate.
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