If you allow members to vote a comment up, it may encourage further contributions. It may also allow the best material to rise to the top.
If you allow members to vote a contribution down, it discourages contributions. Even worse, it may breed dislike between people.
This is a variation of the attribution bias. If someone votes your contribution up, you assume it's because it's a good comment and you're a clever person for posting it. Each positive vote reinforces this view. You deserved that positive vote.
However, if someone votes your contribution down, you assume it's because the person is vindictive or somehow damaged. As a result, you're more likely to take revenge by voting down any future contributions from that person. This breeds unnecessary conflict.
Worse still, members fear rejection more than they want validation. They fear making contributions because it might be voted down. They participate less. A community professional recently told me about a sharp rise in contributions after removing negative voting.
There is no real use for negative voting. Members can ignore contributions they don't like (or simple disagree with their own contributions). They can report comments directly that are against the rules. Positive voting lets the top contributions rise to the top naturally.
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