You can tell at four months if your child will be a "Clint Eastwood" type, according to Jerome Kagan, a retired Harvard psychologist. His new book ("The Long Shadow of Temperament") is outlined in this Boston Globe story.
"At 4 months old, plop your baby into a bouncy seat and present him with a series of colorful new toys - ones he’s never seen - one after the other, for 20 seconds at a time. Does he cry madly and shake his arms and legs? If yes, be forewarned: Your baby may be at higher risk for ’developing serious anxiety over social interactions’ a decade down the road.
If he screams at 4 months, he’ll be more likely to stay home from junior-high dances. If he screams, he’ll be more likely to answer ’no’ when a psychologist asks, at age 11, ’Are you happy most of the time?’
It won’t really matter if you cuddled your child as an infant or showered him with play dates as a toddler. He’ll probably never be a brash CEO or politician, although he might become a brilliant solitary researcher or a melancholy poet.
On the other hand, if your baby just stares calmly at the toys, he will be calm on dates but also slightly more likely to become a delinquent, because parental threats won’t faze him."
There is a lot more (Pinker, Bowlby’s "attachment theory," etc.)
and the article, although not deep, is just good and long enough to be interesting. It must be said that it is fun watching people limit their thinking by depending on modern science.