Smiling is a great icebreaker between strangers. Showing the pearly whites is an invitation to engage. But smartphones swallow smiles.

A recent study by American and Canadian researchers shows that strangers keep their smiles in check when they have their smartphones handy, curtailing human interaction.

This has implications for human health since scientists are increasingly recognizing the connection between social isolation and illness, including a heightened risk of cardiovascular disease.

To see how phones affect social interaction, researchers paired strangers in a room for 10 minutes, either with or without their smartphones. The participants, nearly 170 in all, were videotaped.

The researchers say the smartphone group was less likely to engage in conversation and smiled less frequently — 103 smiling seconds compared with 149 seconds for those without their phones.

Furthermore, the smartphone group displayed genuine smiles, also called Duchenne smiles, about 30 percent less frequently, the researchers found. These are smiles that raise the corners of the mouth and the cheeks, producing crow’s feet around the eyes.

The smartphones, researchers say, may signal disinterest to the other party in the room, making each less likely to smile. Or the phone might be nothing more than a distraction.

Scientists point out that smartphones can offer a psychological benefit, allowing the user to retreat into the digital world during a caustic social interaction. In that case, they say, the phone works like a security blanket.

But if you want to make a new friend, or just brighten someone’s day, put the phone down and look up. And don’t forget to smile.