A researcher from Ste. Justine here in Montreal, Mara Brendgen did a really interesting piece of research about friendship as correlated with depression in children.  You can read the article (if you have academic access) or a nice summary of it at BPS Research Digest by Christian Jarrett.   By studying kids who have an identical twin with depression and kids with a fraternal twin with depression Brengdan and her colleagues were able to identify kids who were genetically pre-disposed to depression but not depressed themselves.  Then she looked at those kids and looked at the quality and quantity of their friendships.

There was a beautiful piece on This American Life last week about a girl who had something like Asperger's.  Her stereotypical conversation and poor social reciprocity get in the way of friendships.  But she still wants friendships.  Eventually she becomes angry and aggressive because she is so lonely.  Her moms take her to all sorts of specialists (a humbling note for those who work with kids with learning and/or developmental disorders) and none of it really helps until... she makes a friend, a friend who is interested in the same things as her.   (It starts at about 41:00 minutes)

A question I have about Brengden's research (or Jarrett's summary, not sure which) is cause and effect.  While kids who are genetically pre-disposed towards depression may have fewer friends or friendships they value less, is that a cause or an effect or are they related through some other factor such as personality type or attachment style? 

There is a correlation between people with schizophrenia and social isolation which has lead to the recommendation (here, for example) that people at high risk of schizophrenia make a conscious effort not to self-isolate.  But we all know that correlation doesn't mean causation or else we would all demand more importation of Mexican lemons until highway deaths were eliminated (link to this and other bizarre correlations). 

Nurturing a kid's positive friendships and encouraging him/her to view friendships as worthy of investing some time and energy in seems to me like a good idea whether or not they are at elevated risk of mental illness.  Of course, for a kid who is less good at friendship or less interested in it placing a lot of emphasis on making friends can backfire by making him/her feel more incapable.  Socially awkward kids often view friendship as an ability akin to drawing or music that relies on a high degree of innate talent (it is interesting that many kids who feel bad about their ability to make friends do so around the age of nine or ten when they also start to notice that other kids are way better than them at some things without having to try hard).  It may be true that some combination of genes and very early wiring can make a person better at social situations than others, but I tell kids that friendship is more like riding a bike than playing music; it is something you can learn at just about any age, something you can always get better at by persevering and something you will never be able to learn do by watching others.  You need to try it, fall down and try again.