Does obesity spread like a virus through networks of friends and friends of friends? Do smoking, loneliness, happiness, depression and illegal drug use also proliferate through social networks?

Over the past few years, a series of highly publicized studies by two researchers have concluded that these behaviors can be literally contagious — passed from person to person. And there was an important public health corollary, the researchers said: It should be possible to curb a behavior like obesity by focusing on small groups of people who would then influence their networks.

But now those surprising conclusions have drawn heated criticism from other scientists who claim that the studies’ methodology was flawed and the original data completely inadequate to estimate the role that contagion might play in the spread of these behaviors.

“I know that many professional statisticians felt it was all bunk from the word go,” said Russell Lyons, a mathematics professor at Indiana University, who recently published a scathing review. of the work on contagion of social behaviors.