LOS ANGELES – Whether it is “Women and children first” or “Every man for himself” in a shipwreck may depend on how long it takes the ship to sink, researchers said Monday.

When the Lusitania was torpedoed by a German U-boat in 1915, it sank in 18 minutes and the bulk of survivors were young men and women who responded to their survival instincts.

But when the Titanic struck an iceberg in 1912, it took three hours to go down, allowing time for more civilized instincts to take control. The bulk of the survivors were women, children and people with young children.

Economist Benno Torgler of the Queensland University of Technology in Australia and his colleagues studied the two sinkings in order to explore the economic theory that people generally behave in a rational and selfish manner. The two tragedies provided a “natural experiment” for testing the idea, because the passengers on the two ships were quite similar in terms of gender and wealth.

The primary difference was how long it took the ships to sink. Reporting in the Proceedings of the National Academy of Science, the researchers found that on the Titanic, children had a 14.8 percent higher probability of surviving, a person accompanying a child had a 19.6 percent high probability and women had more than a 50 percent higher probability. On the Lusitania, in contrast, fit young men and women were the most likely to make it into the lifeboats.

Social class was also important. On the Titanic, first-class passengers were about 44 percent more likely to survive, while on the Lusitania, passengers from steerage were more likely to emerge safely.

The authors concluded the most likely reason for the differences was the amount of time passengers had to effect escape. They suggested that when people have little time to react, gut instincts may rule. When more time is available, social influences play a bigger role.

“The Lusitania sank so quickly, and half of the lifeboats couldn’t even be used,” said Ed Kamuda, president of the Titanic Historical Society in Indian Orchard, Mass. “The younger crowd would be able to make it into the boats, could jump into them as they swung away from the ship. The Titanic was pretty well on an even keel, and they had all sorts of time.”

But psychologists noted that many factors other than following social norms could come into play in a disaster, including an evolutionary imperative to save the species, attachments that are formed between individuals during the event and the leadership of authority figures.