Specifically:

By applying our methodology to four large online news communities for which we have complete article commenting and comment voting data (about 140 million votes on 42 million comments), we discover that community feedback does not appear to drive the behavior of users in a direction that is beneficial to the community, as predicted by the operant conditioning framework. Instead, we find that community feedback is likely to perpetuate undesired behavior. In particular, punished authors actually write worse in subsequent posts, while rewarded authors do not improve significantly.

In a footnote, they discuss what they meant by "write worse":

One important subtlety here is that the observed quality of a post (i.e., the proportion of up-votes) is not entirely a direct consequence of the actual textual quality of the post, but is also affected by community bias effects. We account for this through experiments specifically designed to disentangle these two factors.

They measure post quality based on textual evidence by spinning up a mechanical turk on 171 comments and using that data to train a binomial regression model. So cool!

When comparing the fraction of upvotes received by a user with the fraction of upvotes given by a user, we find a strong linear correlation. This suggests that user behavior is largely "tit-for-tat".... However, we also note an interesting deviation from the general trend. In particular, very negatively evaluated people actually respond in a positive direction: the proportion of up-votes they give is higher than the proportion of up-votes they receive. On the other hand, users receiving many up-votes appear to be more "critical", as they evaluate others more negatively.

Incredibly interesting article. Must read.

EDIT: Consider myself updated. Therefore, I believe downvotes must be destroyed.