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I have asked the same question half a year ago on Graphic Design but received no useful answer.

According to the W3C (and another question on this site), the contrast ratio between body text and text background should be at least 7:1 for good legibility. Google takes this up in their typography guidelines, but they also maintain that

Text with too much contrast can also be hard to read.

Unfortunately, they don't state what "too much contrast" exactly is, and I was unable to find anything definite on the web.

What is the optimal contrast ratio for best readability – that is, neither too little nor too much contrast – for subjects with normal vision ?

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    $\begingroup$ I think it is hard to state there is one true optimal ratio. People are different in many ways and probably there is a range of ratios that appear to be optimal for different people. $\endgroup$ – Robin Kramer Dec 12 '16 at 12:50
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    $\begingroup$ Of course, @RobinKramer, I thought that would be understood. "Optimal" would be something like the mean and standard deviation of the normally distributed shortest reaction times to stimuli with different contrast ratios. Basic psychophysics. $\endgroup$ – user3116 Dec 12 '16 at 14:56
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    $\begingroup$ Are you referring to digital content or printed materials? In the former case, are you referring to CRT screens, LCD, LED .... I think some background would help here. Also, are you referring to colors or grayscale material? $\endgroup$ – AliceD♦ Dec 12 '16 at 14:58
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    $\begingroup$ @Christiaan Finding the optimal contrast will require manipulating both wavelength and luminance. But I suspect no study has ever attempted to research this, so I will be grateful for anything that you can find. $\endgroup$ – user3116 Dec 13 '16 at 13:32
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    $\begingroup$ User Experience SE has a tag for questions related to contrast. $\endgroup$ – user800 Dec 14 '16 at 14:31
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Short answer
Higher contrast increases readability.

Background
In a series of papers under the umbrella "Psychophysics of Reading" (link is to first paper in the series of five) the authors investigate parameters that affect readability in normally sighted folks, as well as people with impaired vision. These studies seem particularly relevant to your question.

The authors investigated reading rates as an outcome measure for readability. In paper V of the series (Legge et al., 2007) they tested the dependence of letter size and contrast (black-and-white) on reading rate. Rates were highest (about 350 words/min) for letters ranging in size from 0.25° to 2° visual angle. Within this range, reading rates proved to be tolerant to contrast reduction. For 1° letters, reading rate decreased by less than a factor of two for a tenfold reduction in contrast. The results were very similar for white-on-black and black-on-white text. Reading rate declined more rapidly for very small (<0.25°) and very large (>2°) letters with lower contrast.

Based on these findings, the intuitive answer to your question is, the larger the contrast, the better. I've never heard of decreasing contrast enhancing reading abilities, barred under some very specific conditions of ultra-low vision, regardless of Google's statements on this. Your first reference clearly supports 'higher contrast is better', without the presence of a sort of optimum.

Reference
- Legge et al, Vis Res (1985); 25(2): 239–52

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