Toronto, May 4 (IANS) Apart from the usual jet lag, the stress associated with air travel also includes air rage -- aggressive behaviour provoked in a passenger on board an aircraft. Initially thought as the result of long flight delays, shrinking seats and a general decline in civility, a new research claims that air rage occurs because of class inequality, that is, the division between business and economy class in the aircraft. "Air rage incidents are more likely when the plane has a first class cabin," said lead researcher Katy DeCelles, associate professor at the University of Toronto in Canada. The findings showed that simply having a first-class compartment made an air rage incident nearly four times more likely, equivalent to the effect of a nine-hour flight delay. Also, the odds increase when economy passengers have to pass through first class to get to their seats, reinforcing the inequality. In addition, the bad behaviour was higher not only for economy passengers, but for those in first class too. Other factors such as crowdedness, alcohol consumption and long flights can also contribute to disruptive incidents, the researchers said, but added that their impact was smaller than one might expect. The study, published in the journal of Proceedings of the National Academy of Sciences, have implications for any physical environment where differences in class or status are apparent -- such as a tiered stadium or a workplace where lower-level employees have to pass by executive offices to get to their cubicles. Further, it also shows that even temporary experiences of inequality can have negative effects. The team used a database of thousands of documented disruptive flight incidents over several years for a large international airline. The incidents were serious enough to be considered a threat to onboard safety, such as passengers refusing to sit down, yelling obscenities at a flight attendant or interfering with smoke sensors so they could sneak a cigarette. Airlines can reconsider ways to reduce negative behaviour between different passenger groups by using a dual gating system, the researchers suggested. "The more you can use those dual gates to board airplanes, separating the first-class cabin from the economy cabin, you're going to have less air rage in both cabins," DeCelles concluded.