  Wikipedia is one of the most successful collaborative projects in history. It
is the largest encyclopedia ever created, with millions of users worldwide
relying on it as the first source of information as well as for fact-checking
and in-depth research. As Wikipedia relies solely on the efforts of its
volunteer-editors, its success might be particularly affected by toxic speech.
In this paper, we analyze all 57 million comments made on user talk pages of
8.5 million editors across the six most active language editions of Wikipedia
to study the potential impact of toxicity on editors' behaviour. We find that
toxic comments consistently reduce the activity of editors, leading to an
estimated loss of 0.5-2 active days per user in the short term. This amounts to
multiple human-years of lost productivity when considering the number of active
contributors to Wikipedia. The effects of toxic comments are even greater in
the long term, as they significantly increase the risk of editors leaving the
project altogether. Using an agent-based model, we demonstrate that toxicity
attacks on Wikipedia have the potential to impede the progress of the entire
project. Our results underscore the importance of mitigating toxic speech on
collaborative platforms such as Wikipedia to ensure their continued success.
