It's one thing for an individual brain to weigh bad news more heavily — it's another for that bias to scale across an entire network. A landmark 2018 MIT study (Vosoughi, Roy, and Aral, published in Science) tracked roughly 126,000 stories shared on Twitter and found that false and negative-toned stories spread significantly faster and reached far more people than true, neutral ones, even controlling for the account sharing them.
Part of the explanation sits in evolutionary biology, not just media design: in social species, an alarm call — a warning about danger — has always carried more urgency to pass along than a report that everything is fine. There's a real survival benefit to spreading a threat signal quickly, and none to spreading the absence of one. That same asymmetry, running through millions of individual sharing decisions, is enough to tilt an entire network toward the negative.
It isn't that people seek out bad news maliciously, or that platforms are uniquely broken — it's that the urge to warn others is old, is real, and scales in ways a single brain's negativity bias never could on its own.