It's late afternoon, and my children and I are playing in the backyard. We're having a fine time. Three-year-old Henry is trying to make the rocket-rider reach high enough for him to touch a branch that hangs above the swing set, while Sam, 7, is poking for roly-poly bugs under the rocks. Then my friend Dinah drives up.

As soon as they see a visitor's car pulling into the driveway, the boys shift out of their comfortable backyard selves into their public, we're-onstage-now roles. It's opening night, and once again Henry's the self-made star of the show. Sam, hanging back behind the curtain, isn't even in the cast.

In his haste to greet Dinah the very moment her car door swings open, Henry nearly kills himself attempting to dismount the rocket-rider while it's still in full flight. Perfectly happy before the encroacher arrived on the scene, Sam, by contrast, glares at the offending vehicle. Suddenly he's a slump-shouldered imitation adolescent, leaning sullenly against the swing set and clearly dreading the summons he knows I'm about to give.

I know that Sam's toe-in-the-dirt, mumble-into-the-collar, one-syllable response to social obligation is not shyness. It's more like reserve. He gets along well with his classmates and almost always receives more invitations to play than he can accept in a single weekend. But he refuses to think of someone as a friend just because friendliness is the feeling expected of him in a social situation. Even when he was a baby, strangers would lean into his stroller and make goofy faces at him, but Sam always considered them gravely for a moment before deciding whether to laugh.

Henry was exactly the opposite sort of baby. He bestowed his first social smile at 3 1/2 weeks, his first laugh at barely 2 months. Now he routinely climbs into someone's lap after 30 seconds' acquaintance. He blows goodbye kisses to checkout clerks and gives a jaunty wave to the school crossing guards. People I could swear I've never seen before greet him by name as we stroll through the mall. Is it any wonder that Sam, already reticent by nature, tends to hang back in the shadows while adults make a huge deal over his twinkle-eyed brother?

I always nudge Sam to greet my friends when they drop by, but in truth, I know exactly how he feels. I, too, take a little while to warm up to people. And I, too, have a younger brother who always viewed strangers with an open, interested gaze. Whenever our parents threw parties, Billy would walk from knee to knee, sweetly accepting treats like an openmouthed baby bird. Watching him in action from the doorway, I always found myself doing exactly what my oldest son does now: standing in the chill outside of warm conversation, glowering.

Thankfully, I'm long past those days. Now I look people in the eye when I meet them; I smile and make small talk in gatherings of strangers. When circumstances call for adult social behavior, I can behave like an adult. But I still prefer a quiet dinner out with my husband, or an evening in the home of our best friends, over any party. As with Sam, this is not shyness. It's simply a preference for what's familiar and beloved over what's uncertain and new.

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