With a potentially historic winter storm already causing power outages and warnings issued for around 140 million Americans, countless people in the storm’s path have hurried to stores to stock up on essentials.

Pictures of empty shelves at stores nationwide have made their way to social media as the storm began moving east from Texas early Saturday, with reports of freezing rain and snow in Kansas, Oklahoma, and Arkansas.

Images of Trader Joe’s stores in ., and , show vacant aisles. Meanwhile, farther south, grocery stores in , saw a surge of shoppers stocking up on water, non-perishable foods, canned goods, and batteries. Local residents in the area reported that water was sold out at their Harris Teeter.

But is stocking up the right choice for people, or is this another case of unnecessary panic buying? And where lies the line between being prepared and being paranoid?

Hersh Shefrin, a behavioral finance professor at Santa Clara University, notes that this phenomenon isn’t unusual during major weather events or high-stress situations.

“Stocking up for a few days is definitely reasonable,” he says, but once purchases are driven by a “fight or flight” emotional reaction, that’s when it turns into panic buying.

“Emergency preparedness and panic buying are distinct,” Shefrin tells TIME. “In an ideal scenario, when a storm is approaching, people would take sensible, practical steps to prepare. But when fear takes over, they start focusing on worst-case outcomes.”

He adds that panic buying is often strongly influenced by others.

“Once people start panic buying, their impulses take control,” he continues. “For example, if they’re in a store and see an item not on their list but notice its stock is running low, they might think, ‘I should grab this before it’s gone.’”

Shefrin says the best way to prevent real shortages is for consumers to pause and not let their emotions guide their actions.

Ravi Dhar, a behavioral scientist and Yale University professor, explains that the “inherent uncertainty” around this weekend’s storm severity leads people to try to control what they can.

“People tend to be overly cautious and crave control rather than acting out of hysteria,” Dhar tells TIME. He says the “fear of running out feels worse than overspending,” as people can justify to themselves that they’ll need the items later.

“The media’s focus on the storm and constant weather alerts on apps make it feel more frightening psychologically,” he adds, pointing out that this leads people to react based on emotions instead of the actual risk of harm—especially since the media keeps using terms like “historic” to describe the storm.

Recent instances of panic buying include during the , and those stories of are still vivid in Americans’ minds, Shefrin adds. In 2020, consumers also rushed to stock up on cleaning products, disinfectants, hand sanitizers, and face masks.