When I worked as a doorman in New York’s Paramount Hotel, I met a lot of very interesting characters. One was a mounted police officer who looked like a movie actor (with a dashing mustache and a face of the Clark Gable/ Errol Flynn cast).

The handsome mounted officer had been something of a playboy in his day, and he bragged about having had different girlfriends for different activities – one for skiing, one for going to the theater, and so on. Since I was only 19 years old, he was happy to give me advice on how to handle relationships with women.

I’ll never forget one piece of he imparted: “Never tell a woman you love her.” Why would anyone say such a cold and heartless thing to a young and impressionable fellow? (I was almost embarrassed to admit that I had already told my girlfriend “I love you” hundreds of times).

In a paper just published in the June issue of Journal of and Social Psychology, MIT’s Josh Ackerman joins Vlad Griskevicius (University of Minnesota) and Norm Li (Singapore University), to address this very issue. And although all three of these researchers were once graduate students in my lab, I did not, to the best of my recollection, ever pass on the sage advice from my NYPD romance mentor. But their research helps me understand his psychology.

In one of their six studies, the researchers asked students to imagine they had just started a new romance with someone they found “attractive and interesting.” If you were a subject, you’d read a scenario describing lots of things couples do together, including eating meals and meeting friends with your new partner. Half the time you’d read that you and your partner had already been sexually intimate, the other half the time you’d imagine a partner with whom you had not had . One month into the relationship, you’d imagine your new partner saying: “I love you.” Next, you’d be asked how happy it would make you hear this confession of love, on a scale ranging from 0 (“not at all”) to 7 (“very much”).

You’d also be asked about your “sociosexual orientation,” filling out a questionnaire to determine whether you are someone who is unrestricted (who thinks sex without love or commitment is a fun idea) or restricted (someone who only really enjoys sexual in the context of a committed relationship).

Unrestricted men have a strange double standard about hearing a woman say the words “I love you.” If a woman professes love for an unrestricted man before they have had sex, it makes him happy. Why? Perhaps because it is taken a signal that he is better positioned to experience carnal lovemaking in the near future. If on the other hand, a woman tells an unrestricted man “I love you” after they have already had sex, it makes him less happy. Why? Perhaps because unrestricted guys are, like my mounted cop friend, hoping to take the sexual benefit without having to pay a commitment cost.

For more restricted/monogamous men, it works differently. They are, like women in general, happier to hear a partner say “I love you” after sex than before.

Men are generally more inclined to value sex outside a committed relationship than are women. And this helps explain an interesting discrepancy found in another study reported in the same paper. The researchers asked people their perceptions of whether men or women are generally more likely to say “I love you” first. The common perception was that women are much more likely to make such a verbal commitment. But the common perception is wrong. In actuality, Ackerman and colleagues replicated something found decades ago by other researchers – men are quite a bit more likely to actually say “I love you” first.

Ackerman and his colleagues explain the discrepancy between the common perception and the reality in terms of evolutionary economics, and they connected it to sex differences in parental investment, which I’ve discussed extensively in earlier postings.

In case you’re not familiar with the concept, biologists note that females typically are required to invest more resources in their offspring (minimally carrying a fetus and nursing in the case of humans and other mammals). Hence, any decision is costly, and females tend to make such decisions with great care. Male mammals, who have potentially less to lose, are less selective in entering into sexual contacts. When a man says: “I love you” it indicates a likely willingness to invest more than just the time it takes to implant sperm, and implies he will stay around to raise the children. But because such verbal commitments can be broken, women are more suspicious of the accompanying intentions, and will often choose to wait and see whether there are other signs of continuing commitment before risking a .

As Josh Ackerman puts it:

“Saying: ‘I love you’ is a negotiation process; essentially, you’re making an offer. And from an evolutionary-economics perspective, the decision to make that offer is different for men than it is for women. In the romantic marketplace, women want to minimize the risk of selling too low, whereas men want to minimize the risk of not bidding high enough. For men, the biggest mistake would be to not communicate commitment and lose the relationship. For women, the biggest mistake would be to impulsively trust her partner’s declaration of ‘I love you’ and gamble on a sexual relationship without the man’s investment.”

Back to the handsome mounted New York City cop who advised me never to say “I love you” to a woman. Living in New York City in the 1960s, with a giant population of single available women, and a new spirit of sexual freedom, this leading man on a tall horse was able to play an unrestricted strategy more easily than most men. Indeed, other research by Steve Gangestad and Jeff Simpson indicates that handsome men are more likely to adopt an unrestricted strategy, and other research suggests that such strategies are more successful for such men when there is a high ratio of available women.

For most regular-looking guys living in places where the desirable women have men quite willing to commit to them, withholding love and commitment might be a formula for celibacy. So better advice to all you regular guys, when you start falling head over heels for a woman, is to go ahead and say “I love you” (but only if you mean it, of course).

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Doug Kenrick is author of Sex, Murder, and the Meaning of Life.