New research indicates emotions are contagious — even in the digital world of Facebook.

Researchers at the big blue social network conducted an experiment on nearly 700,000 of the network’s users, without their knowledge.

They removed emotionally positive posts from one group of users and removed emotionally negative posts from another. Any such emotionally loaded posts had between a 10 percent and 90 percent chance of being omitted from user timelines.

The researchers found the behavior of their subjects matched their altered timelines. Those who saw more positive posts posted more positive updates of their own, and vice versa.

In their paper, published this month in the Proceedings of the National Academy of Sciences, they explain: "In contrast to prevailing assumptions, in-person interaction and nonverbal cues are not strictly necessary for emotional contagion."

It's not the first study to find emotion propagates through Facebook like contagious disease — there was another one earlier this year published in the medical journal PLOS One that found similar results.

But it's not just the findings that are getting attention. Facebook is drawing fire for not letting its users know the social-networking site was altering timelines and conducting a study.

Facebook says in its Terms of Service it reserves the right to use user-submitted data "for internal operations, including troubleshooting, data analysis, testing, research and service improvement."

So by its own Terms of Service, Facebook’s research appears legal. But the ethics are a bit cloudier — and are being criticized as tone-deaf and unsettling.

At PandoDaily: "The whole point of the News Feed algorithm, to hear Facebook tell it, is to give users an experience tailored to their wants and interests. Clearly, that objective falls by the wayside anytime Facebook wants to turn its user base into a science experiment."

University of Maryland technology and law professor James Grimmelmann told Slate: "If you are exposing people to something that causes changes in psychological status, that's experimentation. This is the kind of thing that would require informed consent."

And take The Atlantic's headline: "Even the Editor of Facebook's Mood Study Thought It Was Creepy."

Facebook, for its part, told The Atlantic it’s only doing the research to improve its services.