Parents are often confounded when their child's behavior suddenly shifts. They wonder, Why does my son become Satanic the moment he sees my best friend's kid? Or, Why does the teacher praise my daughter's compliance when she's impossible at home? The issue here is an allegedly "good" child going "bad," or conversely, a "bad" child going "good." The truth, as always, is much more complicated.
Let's start with the basics. Though it's easy to think of your precocious darlings as little adults with the ability to question, monitor, and control their emotions, children are actually sensitive blobs of exposed neurons, highly tuned in to their environment, and reactive to changes in it. This serves a developmental purpose. Their life is one big quest to make sense of the world, and taking everything in helps them accomplish this. But given their limited capabilities and goopy brain structure, they're constantly bumping up against information overload. And while this buildup is not necessarily linear (meaning, it doesn't accrue logically from one moment to the next), it is cumulative (meaning, it adds up). So nearly any combination of new experiences can set off a transformation: New car seat + Gymboree + unfamiliar Whole Foods = Freak-out.
One way in which kids corral this volatility is through adherence to rules. Children find solace in clear and consistent parameters. Rules provide them with grounding guidelines and foster safe exploration. This is why kids tend to be "good" at school—where rules are so obvious as to be posted on the door—and why they tend to "freak out" at birthday parties—where ad-hoc management, bizarre rituals, unspecified outcomes, and cake short-circuit all clarity.
Additionally, the good-bad-bad-good issue is often exacerbated by what I call the parental perspective, in which you and your child see the same events or situations very, very differently. You think you're having a great time visiting with your best friend and her kid, while your child sees you ignoring him, behaving unfamiliarly (cackling, dishing), and forcing him in with a child he may not like. You see your arrival at school pickup as cause for celebration; your child sees it as a sudden intrusion into and cessation of their private world. Wouldn't you react?
So what can you do to moderate the effects? You can start by providing advance notice of situations and your expectations, no matter how ordinary or novel they may be—and a couple of words will do. ("Sephora is a very big store.") You can give your child familiar scaffolding around which to moor their understanding of what's up. And you can provide them with a way to connect personally to what's happening, lending them a sense of control. For example, "I'll pick you up after school today. When I get there, I'll give you two minutes to finish what you're doing, then you can collect your things and lead us out." You can also not expect that this is going to work perfectly on the first—or tenth—try. And you can stop always looking for causality ("It was Tuesday, and I was wearing that glittery blouse...."). It's often not there, not where you're looking, or irrelevant, making it a red herring in your Sherlock Holmes stew. And no one likes herring.
