So as early as we have measured this, one in five people has a mental disorder that already hampers their life, and it predicts a hampered life as an adult.
The authors point to some correlations:
"Sociodemographic and psychosocial correlates included persistent poverty beginning in early childhood, limited parental education, low family expressiveness, stressful life events, and violence exposure. Finally, diagnostic status was significantly associated with poorer social competence and family burden."
And of course, knowing my prejudices, you have to expect that I think the demographics have a good chance of being epigenetic factors, and I do. It's all in the chemistry and the synapses that fire.
So what does this mean to child development people? One in five kids in your circle time has something going on in their* brain that is already impairing their life and probably will forever. And that's in addition to the ones who will develop schizophrenia and depression and alcoholism. The 1 in 5 are the ones who are already screwed up.
One thing that makes it hard is that externalizing and internalizing disorders look a lot like normal 3 and 4-year-old behavior. So is it the 1 in 5 who are the most extreme in their 4-year-oldness? Biting is "normal" behavior, though we try to stop it, but not every kid bites. Maybe if we kept track of all our kids' behaviors and got these researchers to give them the diagnostic tests, and tracked them into adulthood, we'd find that biters end up as adult biters, metaphorically speaking.
Or maybe not. How does a teacher in a classroom know if she's dealing with a kid in a bad mood or a future depressed mom? I don't think she does. Oh, sure there are some kids who are just mean, but is a kid who likes to break things going to end up an adult breaker of things? Is the really shy kid going to end up a recluse? I think if you look at bunches of kids, the behavior overlap between normal and the 1 in 5 will be enough that you can't pick out more than some of the kids with the problems, the extreme ones.
And even if you could, what would you do? Suppose you knew a biter had some diagnosed mental disorder of which the biting was an expression. Do you try more aggressively to stop the biting or less aggressively? Or the same? Or should the parents drug the little monster?
I have more to say, but it's time to get ready for work. I wish I had a better record of getting back to ideas.
*Yes, I embrace the singular "they." What's it to you? I also split infinitives at will and end sentences with prepositions.