get kicked out of it on social media. >> for a lot of 13-year-olds, they really have one social group. and if they are left out of that one group, that feels devastating. they also view it as all or nothing. you're popular, in, cool. or you're nothing. you're trash. you're left out. you're excluded. they feel like it will last forever. >> do people ever post photos to make people feel left out on purpose? yes, that actually happens a lot. >> nearly half of the teens in this study said they felt purposely excluded by friends online. it's often many of those same kids that retaliate. more than one-third in this study admitted they purposely exclude others as well. >> it's really powerful form of aggression because it's so subtle that it's considered bad form to respond. so lots of us have experienced the pain of it. many who do it are doing it for the purpose of hurting others but they can do it with the full expectation they'll not pay one single social consequence. >> our study found the biggest source of online conflict for