Explanations aside, researchers unanimously expressed hope that the trends would persist. They noted it was crucial to continue efforts to understand the reasons for the decline, as well as to discourage drug use.

Though smartphones seem ubiquitous in daily life, they are actually so new that researchers are just beginning to understand what the devices may do to the brain. Researchers say phones and social media not only serve a primitive need for connection but can also create powerful feedback loops.

“People are carrying around a portable dopamine pump, and kids have basically been carrying it around for the last 10 years,” said David Greenfield, assistant clinical professor of psychiatry at the University of Connecticut School of Medicine and founder of The Center for Internet and Technology Addiction.

Alexandra Elliott, 17, a senior at George Washington High School in San Francisco, said using her phone for social media “really feels good” in a way consistent with a “chemical release.” A heavy phone user who smokes marijuana occasionally, Alexandra said she didn’t think the two were mutually exclusive.

However, she said, the phone provides a valuable tool for people at parties who don’t want to do drugs because “you can sit around and look like you’re doing something, even if you’re not doing something, like just surfing the web.”

“I’ve done that before,” she explained, “with a group sitting around a circle passing a bong or a joint. And I’ll sit away from the circle texting someone.”

Melanie Clarke, an 18-year-old taking a gap year and working in a Starbucks in Cape Cod, Mass., said she had virtually no interest in drugs, despite having been around her. “Personally, I think it is a substitution,” Ms. Clarke said of her phone, which she said she was rarely without. Ms. Clarke also said she thought the habits depended on the person. “When I’m home alone, my first instinct is to go for the phone. Some kids will break out the bowls,” referring to a marijuana-smoking device.