  Today, participating in discussions on online forums is extremely commonplace
and these discussions have started rendering a strong influence on the overall
opinion of online users. Naturally, twisting the flow of the argument can have
a strong impact on the minds of naive users, which in the long run might have
socio-political ramifications, for example, winning an election or spreading
targeted misinformation. Thus, these platforms are potentially highly
vulnerable to malicious players who might act individually or as a cohort to
breed fallacious arguments with a motive to sway public opinion. Ad hominem
arguments are one of the most effective forms of such fallacies. Although a
simple fallacy, it is effective enough to sway public debates in offline world
and can be used as a precursor to shutting down the voice of opposition by
slander.
  In this work, we take a first step in shedding light on the usage of ad
hominem fallacies in the wild. First, we build a powerful ad hominem detector
with high accuracy (F1 more than 83%, showing a significant improvement over
prior work), even for datasets for which annotated instances constitute a very
small fraction. We then used our detector on 265k arguments collected from the
online debate forum - CreateDebate. Our crowdsourced surveys validate our
in-the-wild predictions on CreateDebate data (94% match with manual
annotation). Our analysis revealed that a surprising 31.23% of CreateDebate
content contains ad hominem fallacy, and a cohort of highly active users post
significantly more ad hominem to suppress opposing views. Then, our temporal
analysis revealed that ad hominem argument usage increased significantly since
the 2016 US Presidential election, not only for topics like Politics, but also
for Science and Law. We conclude by discussing important implications of our
work to detect and defend against ad hominem fallacies.
