More people will be flying this summer than ever, and they’re going to be squeezed into tighter and tighter spots.

Airlines for America, the industry’s lobby, expects a record 234.1 million people will be flying this summer, up an estimated 4 percent compared to last year. Meanwhile, the airlines continue cramming more seats onto their aircraft. It seems intuitive that the more passengers airlines cram into an aircraft, the more aggressive those passengers become. Science suggests that’s the case.

More than 50 years ago, John B. Calhoun, a researcher at the National Institute of Mental Health conducted experiments with rats and mice that demonstrated the ill effects of crowding. When crammed into one place with nowhere to go, the animals became hyperaggressive and even violent, or morbidly withdrawn. People can act the same way, he argued.

Daniel Stokols, a professor of psychology and social behavior at University of California at Irvine, witnessed those stresses firsthand during a coast-to-coast flight. If anything, airlines may be contributing to the psychological conditions that aggravate people’s stress.

“People are like cattle being squished together, to get as many people on that plane,” Stokols said. “And so tempers can flare.”

Stokols cautions against drawing close parallels between animals and humans, and it should be noted that Calhoun’s experiments were focused on entire populations, not the sort of temporary crowding people endure on trains or planes. But there are still valid conclusions to be drawn from those animal studies that explain why passengers and flight crews act out on an overcrowded plane, especially when considering nuances.

On flights, people do not find themselves surrounded by friends. They’re cramped. They can feel vulnerable to contracting diseases from the sniffling and coughing passengers around them.