Could you firm up "known to be" with something a little more definitive? How is this known?

Homeschooled children are also known to be independent, self-assured, outgoing, resilient, and comfortable in real-world social situations not just age-monocultures-with-clear-tasks-and-constant-adult-supervision.

Maybe it's better not to just state positive and negative qualities but rather refer to something more systematic? (Peaceful Dave replied to you with a link to one possible source.)