I have, over the years, had several parents bring their kids to see me because the kids are "shy" or "quiet" or "unwilling to spend time with the family" or even "anti-social". Many times, however, I notice that the kids are not suffering from psychological issues, they're just introverts living in a very extroverted world. This is typically a problem when the parents are extroverted and the child is not. So, just to normalize introversion, here's some information about both introversion and extroversion.
Extroverts are people that enjoy being in groups. They get energized with group activity so the more people there are, the more energized they become. If you think about a concert or other occasion where there are thousands of people, they are the ones that are singing, dancing, making friends with everyone around them and are the last to leave the venue.
Introverts are people that value their privacy and solitude. They get energized by being alone so the more people there are around them, the more drained they become. If these folks are at a concert, well, they probably wouldn't go to a concert at all because of the crowds. It would be too overwhelming (at least without using some coping mechanisms).
Then there are "ambiverts". These folks are somewhere in-between the two extremes. Sometimes they're excited to go out and be in large groups. Sometimes they prefer to hang out at home for a quiet evening away from it all. These folks have the best of both worlds. When life calls on them to be extroverted, they manage quite well. When life allows them to be introverted, they're comfortable there, too.
The "problem" of introversion is that, especially in American society, extroversion is highly regarded. In the school system as well as the business place, people are expected to work well with others in groups--maybe not all of the time, but it is an expectation that they be able to do it and be comfortable with it. Networking and making contacts is a great way to get ahead in business, opening doors to future projects and advancements in one's career. For the introvert, this is something completely uncomfortable. When parents notice that their child does not have many friends, preferring to have one or two close friends instead, or when they complain about going to school or go grudgingly only to hole up in their rooms at the end of the day, that's when I get the phone call--"something is really wrong with my kid! They're depressed!"
I am not so quick to decide that a child is depressed just because she doesn't want to hang out with her family or just because he only has one friend with whom he only texts and only rarely actually sees. To me, my first thought is "maybe he is just more introverted than the rest of his family." Of course, in our work together, a better determination can be made as to whether the child is actually depessed, is suffering from social anxiety, or is simply more on the introverted side. Being introverted is perhaps the hardest to "treat" because it is a personality type. These folks are actually fairly comfortable being by themselves and don't really see much of a need to be otherwise. So my work becomes practicing with them ways to cope when they don't have a choice. There are times when we all MUST do the social thing. The real work comes with having the parent understand the child better, though, and helping everyone, including the introvert themselves, not view the introverted personality as one that is defective and in need of "treatment".
I stumbled upon this clever set of graphs (which of course are not "scientific" in any way, but are pretty accurate in understanding introversion and what its like to be introverted.) A graphic I frequently share with my clients and their parents is this one that helps everyone understand the needs of the introvert.
Basically, I just want to really emphasize that although introversion is not prized in American society, it definitely is important and there's nothing "wrong" with it. Introverts contribute significantly to our society and are an important part of our community. We need to recognize their value and the special set of skills in which introverts outperform their extroverted colleagues. A great resource in learning more about the value of introversion is this book by Susan Cain entitled simply "Quiet".
So, if your child is more comfortable being alone, reading or listening to music in their room with the door closed, or doesn't seem to have many friends, think about the possibility that they're introverted. If you ask them if they WANT to go out and do things like hang out at the mall or go to a big party, don't be overly surprised if they say "no" or, because they're great at knowing that extroverts rule the planet, they might say "yes". It can be hard to distinguish introversion from depression or anxiety. These kids may already be buying into the belief that there's something wrong with being introverted. If you have any concerns at all, though, your best course of action is to bring them into therapy, if nothing more than to rule out depression and anxiety and, if their self-esteem is low (because they think there's something wrong with them) a good therapist can help them see their own value and normalize introversion as well as teach them coping skills for the times that they need to function in the extroverted world in which we live.