Quote:I'm tempted to think that association of dark with dirty may also be a strong reason for that
This is a loose evolutionary psychology argument, and I don't think it holds water.
Of course being dirty can make the skin look darker, but isn’t this effect worse for fair skinned people? The colour prejudice against dark skin occurs even after knowing
that a particular dark person is cleaner than a particular white skinned person. In other words, people in most cultures have these preferences for lighter skin in general, not just for specific individuals. Moreover, it just doesn't make sense that we evolved a genetic predisposition for light skin because of an inability to differentiate between dirty skin and dark skin. We evolved most of our genetic preferences over millions of years in Africa, when all humans were dark. A very dark skinned person who hunts on the grasslands and deserts wouldn't look darker when dirty and covered in dust. In fact, s/he would probably look darker when washed and clean!
Quote:accumulation of dirt on skin makes skin tone darker and washing restores the fairer tone so , having a hygienic lifestyle may be linked with fairness ?,
From a genetic point of view, sure, unhygienic people are less desirable. However, hygiene is usually not related to temporarily being covered in soot etc. It is connected to habitual aspects of cleanliness. In fact, a genetic predisposition to think of light-skinned as hygienic would actually be disadvantageous, because it would result in false positives.
But culturally (as opposed to genetically) hygiene is more directly related to economic well-being and education. It may indeed be true that most people who are less hygienic in India are dark, because of economic reasons. The poorest sections of society are on average darker than the more wealthy sections, and the former are probably less hygienic. This is probably the reason why even you and I subconsciously associate less-hygienic with dark skinned people. In other words, cultural reasons (nurture) are far more powerful than genetic reasons for this association. It is a product of the feedback loop I mentioned in my comment above.
Quote:I thought tanned look is attractive because for pale tones , it makes people took healthier ,that may be the reason for majority preference but of course people can have their individual preferences and also have dark olive-chocolate colored skin as preference
Just the way the sentence is formulated demonstrates the cultural bias against dark skin, depicting preference for it as anomalous. Yes, having a little colour in one’s skin is considered healthy, but why would having a lot of colour be “individual preference” instead of more healthy or less healthy?
Moreover, you can easily demonstrate that the healthy-tan idea which is based on fact is in reality superseded by cultural factors. For example, in South India or Africa where darker skin should be healthier (because melanin is more useful in places where the UV index is high- see below), the preference for light skin is actually greater. And if we want to talk about pale skin, there are cultures where there is preference for it over tanned skin. In the Middle East, the Arabs highly value pale skin over tanned skin (or even over golden skin).
The evolution of skin-color differences in humans is fairly well understood. It occurred through the interplay between two selective forces.
1. Protecting the skin from harmful UV rays.
2. Allowing enough UV rays to pass through to provide essential vitamin D.
In evolutionary biology, such opposing selective forces that have varying effects on different populations are called “clines”. The effect these clines have is to form a distribution of a particular trait (a common example used is size differences in different populations of fish). Certain “clines” can be strong enough selectively to create genetically isolated populations (populations that do not interbreed because of some physical barrier). This can even lead to speciation- formation of two distinct species.
In the case of selection during skin-colour evolution, the clines are geographical. Near the equator, UV radiation is high and melanin production is advantageous. The further away from the equator you go, there is less UV radiation. UVB is required to make vitamin D (one form of it), an essential vitamin. So populations leaving Africa slowly lost their pigmentation in order to remain healthy. This is why there is a natural gradient from dark to light as you go from the equator to the poles.
Despite this, you see today that in Northern Europe, where light skin is more evolutionarily advantageous, people actually prefer tanned skin. Other cultural factors come into play as well. Today we can supplement our diet with vitamin D and therefore do not need pale skin to harvest vitamin D from the sun, thus significantly lowering our chances of getting skin cancer (and ageing the skin as well). Today there are hundreds of millions of people of European origin getting tans out in the sun and in tanning salons, putting themselves at risk. The cultural notions of beauty are a much stronger force than inbuilt genetic preference biases, when it comes to skin colour. In addition, as stated above, when it comes to the majority of people on the planet, cultural factors such as social dominance and institutionalized prejudice are much more explanatory for why the vast majority of people prefer light skin over dark.
The genetic arguments for perpetuation of skin-colour prejudice fail because they are simply not grounded in science but rather in bias. This is similar to how for a long time women were considered less intelligent than men and many genetic arguments were made by scientists, only to eventually discover that the reasons why we had been thinking of women as less intelligent were all cultural. Once the bias is present in culture, it is reinforced through social practice. Many generations of scientists (and men in general) telling people that women are less intelligent contributed to the bias, reinforcing it and resulting in perpetuation of the bias and its expression in various forms.