How can you tell if your child is an introvert? There’s no definitive test you can give your child to make that determination. Your own knowledge of how your child thinks and acts is much more helpful. But scientists have found that studying babies can offer insight into which ones will be more introverted than extroverted.

In her book Quiet, author Susan Cain refers to a study that scientists performed on a group of four-month-old babies; they later followed up on these same children throughout childhood. Using visual observations as well as measurements of heart rate, blood pressure and other physical signs, they determined that how a baby reacts to new experiences can predict whether he will lean toward introversion or extroversion:

The infants heard tape-recorded voices and balloons popping, saw colorful mobiles dance before their eyes, and inhaled the scent of alcohol on cotton swabs. They had wildly varying reactions to the new stimuli. About 20 percent cried lustily and pumped their arms and legs. Kagan called this group “high-reactive.” About 40 percent stayed quiet and placid, moving their arms or legs occasionally, but without all the dramatic limb-pumping. This group Kagan called “low-reactive.” The remaining 40 percent fell between these two extremes. In a startlingly counterintuitive hypothesis, Kagan predicted that it was the infants in the high-reactive group—the lusty arm-pumpers—who were most likely to grow into quiet teenagers.

Later studies of these same babies, now older children, found that many turned out just as predicted:

The high-reactive infants, the 20 percent who’d hollered at the mobiles bobbing above their heads, were more likely to have developed serious, careful personalities. The low-reactive infants—the quiet ones—were more likely to have become relaxed and confident types.

As Ms. Cain said, the results seemed counter-intuitive. But when you consider that introverts tend to be more sensitive, while extroverts like noise and lots of stimuli, it makes sense.

What this means for our children is that they’re wired to be where they are on the introvert-extrovert scale. Trying to change them is not only pointless, but can be harmful. Letting them be who they are and providing them with an atmosphere where they can learn, how ever they prefer to learn, is optimal.

If you take this information and look at your own children, consider that whether a child is introverted or extroverted is not always a clear-cut issue. Most people lean in one direction but may have a few characteristics from the other side.

That’s especially true of adults, by the way; I’ll explain why in the next post.

Next week: How Schools Try to Convert Introverts to Extroverts