For many relationships, there is a single moment that marks a major turning point toward either a future of togetherness or one that splits into separate roads. And that moment usually involves three little words: "I love you."
In books and movies, this simple sentence may seem full of mystery and romance. But a new study suggests that science and evolution may help explain who, in the real world, declares love first and how each partner feels when he or she hears it. Many of the results defy stereotypes.
Even though most people think that women are the romantics in a relationship, for example, men most often say "I love you" first. And most people are happier to hear those words after having sex with their partners than before - except, that is, for playboys on the prowl for short-term hookups, who prefer to hear it beforehand.
To explain their results, the researchers invoke a time when sex inevitably meant the possibility of pregnancy. It would make sense, in that context, for women to be more cautious about expressing love and more skeptical of declarations about a man's feelings for them.
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.
But two subsequent studies, in which people who ranged in age from their mid-20s to their 60s reflected on their current or most recent relationships, showed that men actually declare love first about 70 percent of time.
In a series of three follow-up studies, people responded to questions about how happy it made them to hear declarations of love. Some imagined being in a fictional new relationship. Others had actually just been told "I love you" for the first time in a relationship in the prior week. Their answers revealed a range of nuances.
If the couple had not yet had sex, for instance, men generally were happier to hear the three little words than women were, the researchers reported in the Journal of Personality and Social Psychology.
After sex, women in particular feel a boost of happiness, Ackerman said, supporting the theory that women tend to prioritize a relationship after pregnancy is possible. Men who were generally interested in long-term relationships were also happier to hear that women loved them after sex than before.
"If someone says 'I love you' after sex, it's a better indicator of how they are actually feeling," Ackerman said. "There is no ambiguity that they are trying to get something else out of it."
The biggest outliers were men who tend to go for short-term flings. For them, happiness dipped upon hearing that the women they had just slept with loved them. Being told they were loved before sex, however, made them truly pleased.
The results suggest that evolutionary impulses may drive people to play dating games, even when their emotions feel genuine on the inside, said Douglas Kenrick, a social psychologist at Arizona State University in Tempe.
"A lot of the ways that evolution influences us don't ever enter the level of consciousness," Kenrick said. "People won't say they like chocolate because it had benefits for our ancestors. They just say they like it. We do what feels right, whether or not people are consciously playing the game."