“But what I learned is that the stories on WhatsApp are common to all the media here,” she said.

What sets WhatsApp apart is speed and reach, Ms. Córdova said. In Brazil, more than 120 million people use the service, which is offered free as part of mobile internet plans (that is, using WhatsApp does not count against people’s data rate). As it does in its other big markets — India, South Africa, Saudi Arabia and much of Europe — WhatsApp functions in Brazil as an all-purpose communications tool. It is used for chatting and joking, for trading photos and memes, for news, for political activity and more.

Because of that centrality, Ms. Córdova argued that the problems on WhatsApp in Brazil were mainly a function of the country’s broken political and media environment.

“For example, we don’t really have public libraries in Brazil,” she said. “We don’t have a lot of sources of what people would consider reliable information — and the lack of good sources of information reinforces their beliefs when they see something false on WhatsApp or Facebook.”

Which is not to say WhatsApp is without tools to rein in the mess. This year, after the mob violence in India — another problem that existed before WhatsApp, and may simply have been amplified by the app — the company instituted rules to limit WhatsApp’s “virality.”

In the past, people could freely forward a WhatsApp message to anyone. Now they are restricted to forwarding a message to 20 “chats,” distinct conversations with an individual or group of up to 256 people. (There are six people in the average group, WhatsApp said.) In India, WhatsApp has an even more restrictive forwarding limit: five chats.

WhatsApp characterized the limits as an experiment. As the company learns more about how the limits affect users’ behavior, a spokeswoman told me, it could introduce additional fine-grain limits.

In an op-ed article in The New York Times last week, a group of Brazilian researchers called for WhatsApp to institute the Indian forwarding limit in Brazil to cut down on false news during the campaign. WhatsApp has declined to do so. The spokeswoman said there was not enough time to change before the election, which takes place on Sunday. (Changing the rules would require people to update their app, a process that takes weeks.)