Avoiding eye contact: when is it shyness and when is it disinterest?

Basically the question. If a guy avoid eye contact when talking to a girl, when does it indicate that he is shy and when does it indicate lack of interest? Or is avoiding eye contact always a sign that a guy isn't interested?

How can I differentiate between the two?
Avoiding eye contact: when is it shyness and when is it disinterest?
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