If those instincts persist in the modern age of birth control, the findings may also offer advice for singles navigating today's dating scene.
"If somebody is saying 'I love you' before sex happens, it probably does pay to be a little more skeptical about it," said Josh Ackerman, a social psychologist at the MIT Sloan School of Management in Cambridge, Mass.
"There are all of these underlying factors that go into this kind of thing that we think is very amorphous and can't be quantified, which is love," he added. "In fact, there are these very specific forces on the willingness to say love and how you feel when people say 'I love you.'"
Social psychologists have long known that men tend to express love first in relationships, even though public perception is just the opposite, and the new project started by confirming those assumptions.
In surveys of 45 people who walked by a street corner, Ackerman and colleagues found that 65 percent of people believed that women usually said, "I love you" first in relationships, while 85 percent believed that women were the first to develop serious feelings.