Why is it easier to see black text on a white background? I'm wondering why it's easier to see black text on a white background than it is to see white text on a black background. I say that it's harder based on life experience (Hate webpages with black backgrounds!) and not from any actual studies I know of that have been done. Nonetheless, I think most people will agree with me on this one.

My two general theories are:


For some reason it strains the eyes (not quite sure why that would be), and
We're just really used to it, so our brains have become wired specifically to process black text on white backgrounds
The short answer is that it is not necessarily easier to read black on white. Contrast is more important in lightness and colour, it just so happens that black and white is the highest contrast. So its no accident that most books are in black and white. There is a plethora of research on perceptual processing which is easily found on google scholar if you which to investigate this further. 

Colour and Reading
In 1984 a study was conducted comparing 24 colour combinations of text and back ground, this was of course on an older monitor, but there wasn't any significant difference between individuals in there ability to detect text targets relative to colour different colour combinations. This included black and white. However research with colour contrast, has found that variations in contrast of colour rather than colour itself influence reading speeds, and errors in reading. Greater colour contrast between text and back ground is reported to enhance speed of reading and reducing errors. In addition improving speed of reading, high colour contrast has been found enhance retention of text based information when compared to lower colour contrasts. 

fig 1. low colour contrast



fig 2. high colour contrast



In addition to colour, contrast in luminescence (lightness) between text and background also influences readability. According to Knoblach et al (2001) high contrasts in luminescence between background and text enhances reading speed to a greater degree than colour contrast, although colour contrast keeps reading speed high when luminescence is low. In other words greater differences in the lightness and the colour of text and background is optimum for reading ability. Industrial researchers have taken this further with a 2009 study by Hewlett Packard (HP) specifically stating that 30 units CIE lightness contrast is optimum for reading rates and reduced error, this is not the highest contrast, although still high. However high colour contract and luminescence contrast are not additive, as they are coded in the same way in the visual system. Essentially so as long one of the contrasts is high reading speed and errors should low, meaning you don't need to use back on white. 

What I haven't discussed here the two systems which process light: the magnocellular and parvocellular systems. So yes we are hardwired to process colour differently from grayscale.

fig 3. Diagram of Visual system



note. Mi = Magnocelluar (grayscale, low resolution, movement) Pi = Parvocelluar (colour and high resolution)

The magnocellular visual pathway which is dominate for processing text, and uses gray scale (black to white). However dyslexics, like myself, appear to have a preference and adaption toward utilising the parvocellular visual pathway (colour processing). This is likely due to differences in visual processing using the magnocellular system between controls and dyslexics, which suggests that dyslexics do not process greyscale as well as non-dyslexics. However this isn't to say that you need to be dyslexic to prefer processing with the colour system than grayscale. THIS SECTION IS SPECULATION BASED ON NEURAL MECHANICS There is also the possibility that prolonged activation of the magnocelluar system, by reading, may fatigue the neurons or/and photoreceptors, consequently reducing ease of reading. Hence why e-readers tend to use grey backgrounds.   

Summary

There is more stuff to add in about positive and negative contrasts, for instance there is little difference in reading ability for black on white in comparison to vice versa. Overall, however, there is little difference in reading performance so long as colour and luminescence contrasts are high, meaning that black on white (highest contrast in both) is not necessarily better. Some individuals have a preference for using colour over grey scale, which while activating different visual pathways is still coded similarly for contrast in the brain. The preference against black on white may be due to the fatiguing nature of over activation of neurons in the visual pathways, or other factors such as dyslexia.